redundant linear indexing

[hexaeder]

I found the offset/index stuff rather confusing and redundant. I think this could be reorganized a bit in order to make it more straight forward. Right now the index information is stored as pairs of offset+lenght as well as ranges in GraphStruct and again decentralized in the EdgeData/VertexData. Here are some ideas:

Each vortex and (in contrast to LighGraphs) each edge is identified by a unique index. Since the GraphStruct should not depend on the concrete underlying data structure. It should only allow quick access to

• index of src and dst vertex for given edge
• indices of all outgoing/incomming edges for given vertex

struct GraphStruct
    v_dims::Vector{Int}
    e_dims::Vector{Int}
    v_syms::Vector{Symbol}
    e_syms::Vector{Symbol}
    _e_src_vert::Vector{Int}
    _e_dst_vert::Vector{Int}
    _v_out_edges::Vector{Vector{Int}}
    _v_in_edges::Vector{Vector{Int}}
end

During the initialisation of GraphData all of the offsets could be calculated explicitly very cheap like

@inline function linear_vert_idx(g::GraphStruct, i)
    offset = i==1 ? 0 : sum(@view g.v_dims[1:i-1])
    @inbounds last = offset + g.v_dims[i]
    offset+1:last
end

... and once the 'views' aka EdgeData/VertexData are constructed they are cached there.