New pushout complement for AttrVars

[kris-brown]

This PR redoes how we handle AttrVars for pushout complements in light of #53

The computation is no longer handled pointwise. Although perhaps we can develop a clean understanding of what should happen in terms of pushout complements in a Kleisli category or in the machinery described by this blog post, for now there is just intuition guided by some concrete examples.

Additional checks are needed to see if a match is valid because, if the match is not monic, it could be possible that two variables (which are mapped to different values in I -> R) are identified with each other.

This is a breaking change because we no longer allow rewriting ACSets which themselves have variables (the rules, of course, still have variables, as well as the intermediate state K in between the initial and final state of a rewrite).

[codecov]

Codecov Report

Attention: Patch coverage is 98.68421% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.79%. Comparing base (690fe70) to head (79d9548).

Files	Patch %	Lines
src/categorical_algebra/CSets.jl	98.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
- Coverage   85.94%   85.79%   -0.15%     
==========================================
  Files          31       31              
  Lines        2020     2063      +43     
==========================================
+ Hits         1736     1770      +34     
- Misses        284      293       +9     

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