Ep/aggregate arithmetic

[potash]

Overloads the arithmetic (+,-,*,/,//) and comparison (<.<=,,==,>,>=) and boolean (&,|) operators. Handles naming of the resulting aggregates.

E.g. Aggregate("x", "sum") / Aggregate("y","count") produces the SQL line

sum(x)*1.0 / count(y) AS "x_sum/y_count"

[mbauman]

Hrm, why isn't Travis running?

[mbauman]

Oh. https://www.traviscistatus.com

[codecov-io]

Codecov Report

Merging #63 into master will decrease coverage by 4.46%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage     100%   95.53%   -4.47%     
==========================================
  Files           4        4              
  Lines         222      269      +47     
==========================================
+ Hits          222      257      +35     
- Misses          0       12      +12

Impacted Files	Coverage Δ
collate/spacetime.py	100% <100%> (ø)	✅
collate/collate.py	93.71% <75.51%> (-6.29%)	❌

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