Update AbstractGraph interface

[dcerkoney]

1. Add setters/getters to AbstractGraph interface
2. Implement FeynmanGraph/Graph conversion functions
3. Remove unnecessary parameteric type C for constants in arithmetic operations

[codecov-commenter]

Codecov Report

Attention: 117 lines in your changes are missing coverage. Please review.

Comparison is base (9a1cb5f) 64.71% compared to head (11207c7) 64.12%.
Report is 13 commits behind head on computgraph.

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Additional details and impacted files

@@               Coverage Diff               @@
##           computgraph     #148      +/-   ##
===============================================
- Coverage        64.71%   64.12%   -0.60%     
===============================================
  Files               62       63       +1     
  Lines             4328     4432     +104     
===============================================
+ Hits              2801     2842      +41     
- Misses            1527     1590      +63     

Files	Coverage Δ
src/FeynmanDiagram.jl	75.00% <ø> (ø)
src/backend/compiler.jl	100.00% <ø> (ø)
src/computational_graph/ComputationalGraph.jl	100.00% <ø> (ø)
src/quantum_operator/expression.jl	96.55% <100.00%> (ø)
src/computational_graph/tree_properties.jl	48.68% <85.71%> (ø)
src/computational_graph/conversions.jl	50.00% <50.00%> (ø)
src/computational_graph/optimize.jl	86.25% <96.49%> (ø)
src/computational_graph/transform.jl	97.36% <90.69%> (+0.08%)	⬆️
src/computational_graph/feynmangraph.jl	86.56% <74.13%> (-4.61%)	⬇️
src/computational_graph/graph.jl	69.34% <75.00%> (+1.02%)	⬆️
... and 3 more

... and 1 file with indirect coverage changes

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