Added a PartialOrder transform

[velochy]

This is an implementation of a partial order transform as discussed in https://discourse.pymc.io/t/feature-proposal-poset-order-transform/16753/7

I have tried to make it as generic as possible:

• It takes only the generating DAG and computes the transitive reduction
• It does not assume the DAG is given in topological order but instead handles sorting internally
• It converts the matrix to an adjacency list of incoming nodes, reducing computation for low-degree orders
• It can either take one single DAG for all data or allows specifying a separate DAG for each row

As I am new to contributing to both pymc and pymc-extras, I will likely need some help to get it acceptable.
So far, I have added the code and tests for it, but I don't know how to handle documentation yet.
Also anything else that might need to happen before this can be merged.

[velochy]

Added to documentation too now. Please let me know if anything needs doing/improving