Rename "Doctrines" module to "Theories"

[epatters]

I propose that the module Catlab.Doctrines be renamed to Catlab.Theories, for several reasons:

1. "Theories" more accurately reflects what is in there, namely a collection of GATs (and default syntax systems for them). Many of these theories are phrased categorically but not all of them, e.g., we will have preorders and posets when Add Preorder and Thin Category Implementation #132 is merged.

2. "Doctrine" has a technical sense in 2-category theory that we are not taking seriously.

3. "Doctrine," as a plain English word, has unpleasant dogmatic connotations. I do not wish to give the impression that we see category theory as a religion.

Granted, this would be a breaking change motivated entirely by bikeshedding, but I still think it is worth it.

[jpfairbanks]

Yes I agree we should do this. @mehalter what do you think about the timing wrt our changes on SM.jl?