Add edge aliasing and edge constraints

[j6k4m8]

This PR adds support for edge aliases (first described in #110) and comparisons between edge attributes with values and with other edges.

This enables syntax like this:

A -> B as ab
B -> A as ba

ab.weight > ba.weight

• [x] Add support in the DSL
• [x] Add support in the parser + transformer
• [x] Add support in the executors:
  • [x] GrandIso
  • [x] NetworkX
  • [x] NeuPrint
  • [x] Neo4j

I am going to push macro support in a separate PR, since this one is getting pretty lengthy already!

[codecov-commenter]

Codecov Report

Merging #119 (db4154a) into master (16e5f84) will increase coverage by 1.04%. The diff coverage is 98.06%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
+ Coverage   90.12%   91.17%   +1.04%     
==========================================
  Files          22       22              
  Lines        1793     2028     +235     
==========================================
+ Hits         1616     1849     +233     
- Misses        177      179       +2     

Impacted Files	Coverage Δ
dotmotif/parsers/v2/__init__.py	94.68% <93.67%> (-0.74%)	:arrow_down:
dotmotif/__init__.py	97.59% <100.00%> (+3.33%)	:arrow_up:
dotmotif/executors/GrandIsoExecutor.py	83.33% <100.00%> (+0.47%)	:arrow_up:
dotmotif/executors/Neo4jExecutor.py	72.61% <100.00%> (+1.01%)	:arrow_up:
dotmotif/executors/NetworkXExecutor.py	92.30% <100.00%> (+0.83%)	:arrow_up:
dotmotif/executors/test_dm_cypher.py	100.00% <100.00%> (ø)
dotmotif/executors/test_grandisoexecutor.py	100.00% <100.00%> (ø)
dotmotif/executors/test_networkxexecutor.py	100.00% <100.00%> (ø)
dotmotif/parsers/v2/test_v2_parser.py	100.00% <100.00%> (ø)

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