Index RVs using labels

[twiecki]

dims and coords are awesome. But we currently can't use them to index into RVs. We could do something like pandas:

with pm.Model(coords={"schools": ["one", "two"]):
    betas = pm.Normal("beta", dims="schools")
    m = x["one"] * betas.loc["one"] + x["two"] * betas.loc["two"]

This could be done with simple overloading of RVs and internally using coords to map to the index.