Implement set_data as a model transformation

[ricardoV94]

Description

I tried to demonstrate the basic functionality of PyMC in this notebook: https://colab.research.google.com/drive/1JyIeu2NMl_z6Y8ZniZn3jDrzzUL3i81q?usp=sharing

I think pm.set_data jumps out as the odd one out that doesn't work with a functional approach but mutates the model inplace. Would be nice to allow pm.set_data with the same API (could introspect the first argument?). Under the hood we can use pm.do which already allows the simple behavior of pm.set_data.

We can also create some function with another name. In either case, we should not break the existing behavior of set_data.

It also seems like some of these model transforms should accept new coords to override the old ones when we want to change the shape of the variables?