Add macro edge aliases

[j6k4m8]

This adds support for complex edge constraints in macros:

decreasing_edge_weights(a, b, c) {
    a -> b as ab
    b -> c as bc
    ab.weight > bc.weight
}

...

In increasing levels of challengingness:

• Add support for simple (edge-value) edge constraints in macros
• Add support for dynamic (edge-edge) edge constraints in macros
• Extend support for recursive calls to macros with simple constraints
• Extend support for recursive calls to macros with dynamic constraints
• Update documentation

This fixes #110 and finishes work started in #119.

[codecov-commenter]

Codecov Report

Merging #121 (b493ac6) into master (16e5f84) will increase coverage by 1.20%.
The diff coverage is 97.15%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   90.12%   91.33%   +1.20%     
==========================================
  Files          22       22              
  Lines        1793     2112     +319     
==========================================
+ Hits         1616     1929     +313     
- Misses        177      183       +6     

Impacted Files	Coverage Δ
dotmotif/parsers/v2/__init__.py	93.93% <92.64%> (-1.48%)	⬇️
dotmotif/__init__.py	97.59% <100.00%> (+3.33%)	⬆️
dotmotif/executors/GrandIsoExecutor.py	83.33% <100.00%> (+0.47%)	⬆️
dotmotif/executors/Neo4jExecutor.py	72.61% <100.00%> (+1.01%)	⬆️
dotmotif/executors/NetworkXExecutor.py	93.00% <100.00%> (+1.53%)	⬆️
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)
... and 1 more

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