[ENH] Add functional submodule

[adam2392]

Related to: #3

Changes proposed in this pull request:

• Adds an API sketch for converting an existing graph to a linear functional Gaussian model

This starts some work in the direction of using networkx as the base-layer to enable another big umbrella in causal inference: functional causal graphical models.

In the future, we will want to be able to add arbitrary complexity to the functions and meaning of the edges (kind of like an SCM) that is constrained to the structure of the graph. We should also be able to "inspect" properties if possible, such as weak-faithfulness in the linear case.

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[codecov-commenter]

Codecov Report

Merging #75 (84ce35f) into main (8658c5b) will increase coverage by 0.17%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   84.42%   84.59%   +0.17%     
==========================================
  Files          34       35       +1     
  Lines        2555     2590      +35     
  Branches      687      698      +11     
==========================================
+ Hits         2157     2191      +34     
+ Misses        251      247       -4     
- Partials      147      152       +5     

Impacted Files	Coverage Δ
pywhy_graphs/functional/linear.py	85.71% <85.71%> (ø)

... and 1 file with indirect coverage changes

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[adam2392]

I will go ahead and merge unless there are objections?