Add transformation/optimization to remove zero-valued subgraphs

[dcerkoney]

1. Add a helper function disconnect_subgraphs!(g::AbstractGraph) to fully disconnect all subgraphs from a graph g
2. Add a tree property has_zero_subfactors(g::AbstractGraph) which returns true if graph g is not a leaf and contains only zero subgraph factors
3. Add transformations remove_zero_valued_subgraphs! and remove_zero_valued_subgraphs
4. Add optimization remove_all_zero_valued_subgraphs!
5. Add the relevant tests
6. Bugfix in function to_dot_str and add html node labels for $\Pi$, $\Gamma^{(3)}$, and $\Gamma^{(4)}$ insertions

[codecov-commenter]

Codecov Report

Attention: 77 lines in your changes are missing coverage. Please review.

Comparison is base (7485eab) 61.39% compared to head (ad2b25d) 61.50%.

Files	Patch %	Lines
src/backend/to_dot.jl	0.00%	76 Missing ⚠️
src/computational_graph/transform.jl	94.11%	1 Missing ⚠️

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Additional details and impacted files

@@               Coverage Diff               @@
##           computgraph     #168      +/-   ##
===============================================
+ Coverage        61.39%   61.50%   +0.10%     
===============================================
  Files               66       66              
  Lines             5010     5065      +55     
===============================================
+ Hits              3076     3115      +39     
- Misses            1934     1950      +16     

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