new LSCM simulation functions

[augeorge]

several updates to NOCAP:

• function to convert a NXmixed graph to a pgmpy bayesian network - corrected for isolated nodes and self loops
• linear structural causal model functions to simulate, calibrate, and intervene.
• function to calculate average treatment effect.

[augeorge]

added additional utility module + tests to deal with:

• parsing GRN from ecocyc,
• converting to DAG,
• non-linear Hill model (SERGIO)
• SERGIO data generation

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