Improve graph I/O

[dcerkoney]

1. Add getter properties and setter set_properties! to AbstractGraph interface. This fixes a bug in I/O for user-defined concrete graph types such as the example ConcreteGraph defined in the test set.
2. Improve string representations for graphs, Feynman graphs, and operators.
3. Add plaintext representations for all custom graph, operator, and quantum operator symbols.
4. Bugfix for typestr in function _stringrep to correctly display subgraph IDs (see examples below).
5. Add tests for all string representations.
6. Clean up comments in tests and tree_properties.jl.

For example, the code

using AbstractTrees, FeynmanDiagram

ComputationalGraphs.uidreset();
g = 2 * (Graph([]) + Graph([]));

function display(g)
    println("plaintext:\n", g)  # plaintext representation
    println("\nREPL:")
    show(g)                     # decorated representation shown by the REPL
    println("\n\nAbstractTrees:")
    print_tree(g)
end

display(g)

now produces the output:

plaintext:
4: Prod(3)=0.0

REPL:
4: Ⓧ (3)=0.0

AbstractTrees:
4: Ⓧ (3)=0.0
└─ 3: ⨁(1,2)=0.0
   ├─ 1: 0.0
   └─ 2: 0.0

instead of

plaintext:
4=0.0=FeynmanDiagram.ComputationalGraphs.Prod 

REPL:
4=0.0=FeynmanDiagram.ComputationalGraphs.Prod 

AbstractTrees:
4 : 4=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
└─ 3 : 3=0.0=FeynmanDiagram.ComputationalGraphs.Sum 
   ├─ 1 : 1=0.0()
   └─ 2 : 2=0.0()

Similarly, the code

using AbstractTrees
using FeynmanDiagram
import FeynmanDiagram.FrontEnds: NoHartree

ComputationalGraphs.uidreset();
para = Parquet.DiagPara(type=Parquet.SigmaDiag, innerLoopNum=2, hasTau=true, filter=[NoHartree,]);
sigma2_ins = optimize!(Parquet.build(para).diagram)[1];
dict_sigma2_AD = taylorAD(
    [sigma2_ins], 
    [1, 0],
    [x -> x isa FrontEnds.BareGreenId, x -> x isa FrontEnds.BareInteractionId],
);
dmu_sigma2 = dict_sigma2_AD[[1, 0]][1];
print_tree(dmu_sigma2; maxdepth=7)

now produces the output

126: Ins, k[1.0, 0.0, 0.0], t(1, 1)[1]=Ⓧ (124)=0.0
└─ 124: Ins, k[1.0, 0.0, 0.0], t(1, 1)[1]=Ⓧ (119,1)=0.0
   ├─ 119: Dyn, k[0.0, 1.0, 0.0], t(1, 1)[1]=⨁(117,118)=0.0
   │  ├─ 117: [1]=Ⓧ (7,111)=0.0
   │  │  ├─ 7: g0, Dyn, k[0.0, 1.0, 0.0], t(1, 2)=0.0
   │  │  └─ 111: (t = (2, 1),)[1]=⨁(109,110)=0.0
   │  │     ├─ 109: [1]=Ⓧ (99,103)=0.0
   │  │     │  ├─ 99: Ins, k[0.0, 1.0, 0.0], t(2, 2)=Ⓧ (14,8)=0.0
   │  │     │  │  ├─ 14: Gfock, Dyn, k[0.0, 0.0, 1.0], t(2, 2)=0.0
   │  │     │  │  └─ 8: ccIns, k[0.0, 1.0, -1.0], t(1, 1)=0.0
   │  │     │  └─ 103: Dyn, k[0.0, 1.0, 0.0], t(2, 1)[1]=0.0
   │  │     └─ 110: [1]=Ⓧ (100,21)=0.0
   │  │        ├─ 100: Ins, k[0.0, 1.0, 0.0], t(2, 2)[1]=Ⓧ (95,8)=0.0
   │  │        │  ├─ 95: Dyn, k[0.0, 0.0, 1.0], t(2, 2)[1]=0.0
   │  │        │  └─ 8: ccIns, k[0.0, 1.0, -1.0], t(1, 1)=0.0
   │  │        └─ 21: G, Dyn, k[0.0, 1.0, 0.0], t(2, 1)=0.0
   │  └─ 118: [1]=Ⓧ (94,108)=0.0
   │     ├─ 94: Dyn, k[0.0, 1.0, 0.0], t(1, 2)[1]=0.0
   │     └─ 108: (t = (2, 1),)=Ⓧ (99,21)=0.0
   │        ├─ 99: Ins, k[0.0, 1.0, 0.0], t(2, 2)=Ⓧ (14,8)=0.0
   │        │  ├─ 14: Gfock, Dyn, k[0.0, 0.0, 1.0], t(2, 2)=0.0
   │        │  └─ 8: ccIns, k[0.0, 1.0, -1.0], t(1, 1)=0.0
   │        └─ 21: G, Dyn, k[0.0, 1.0, 0.0], t(2, 1)=0.0
   └─ 1: ccIns, k[1.0, -1.0, 0.0], t(1, 1)=0.0

instead of

126 : 126Ins, k[1.0, 0.0, 0.0], t(1, 1)[1]=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
└─ 124 : 124Ins, k[1.0, 0.0, 0.0], t(1, 1)[1]=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
   ├─ 119 : 119Dyn, k[0.0, 1.0, 0.0], t(1, 1)[1]=0.0=FeynmanDiagram.ComputationalGraphs.Sum 
   │  ├─ 117 : 117[1]=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
   │  │  ├─ 7 : 7-g0Dyn, k[0.0, 1.0, 0.0], t(1, 2)=0.0()
   │  │  └─ 111 : 111(t = (2, 1),)[1]=0.0=FeynmanDiagram.ComputationalGraphs.Sum 
   │  │     ├─ 109 : 109[1]=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
   │  │     │  ├─ 99 : 99Ins, k[0.0, 1.0, 0.0], t(2, 2)=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
   │  │     │  │  ├─ 14 : 14-GfockDyn, k[0.0, 0.0, 1.0], t(2, 2)=0.0()
   │  │     │  │  └─ 8 : 8ccIns, k[0.0, 1.0, -1.0], t(1, 1)=0.0()
   │  │     │  └─ 103 : 103Dyn, k[0.0, 1.0, 0.0], t(2, 1)[1]=0.0()
   │  │     └─ 110 : 110[1]=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
   │  │        ├─ 100 : 100Ins, k[0.0, 1.0, 0.0], t(2, 2)[1]=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
   │  │        │  ├─ 95 : 95Dyn, k[0.0, 0.0, 1.0], t(2, 2)[1]=0.0()
   │  │        │  └─ 8 : 8ccIns, k[0.0, 1.0, -1.0], t(1, 1)=0.0()
   │  │        └─ 21 : 21-GDyn, k[0.0, 1.0, 0.0], t(2, 1)=0.0()
   │  └─ 118 : 118[1]=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
   │     ├─ 94 : 94Dyn, k[0.0, 1.0, 0.0], t(1, 2)[1]=0.0()
   │     └─ 108 : 108(t = (2, 1),)=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
   │        ├─ 99 : 99Ins, k[0.0, 1.0, 0.0], t(2, 2)=0.0=FeynmanDiagram.ComputationalGraphs.Prod 
   │        │  ├─ 14 : 14-GfockDyn, k[0.0, 0.0, 1.0], t(2, 2)=0.0()
   │        │  └─ 8 : 8ccIns, k[0.0, 1.0, -1.0], t(1, 1)=0.0()
   │        └─ 21 : 21-GDyn, k[0.0, 1.0, 0.0], t(2, 1)=0.0()
   └─ 1 : 1ccIns, k[1.0, -1.0, 0.0], t(1, 1)=0.0()