refactor hardcopy themes

[Alex-Jordan]

This pairs with openwebwork/webwork2#2099 that refactors hardcopy themes.

Part of that refactor is cleanly separating:

• tex packags and macros that supports PG directly
• tex packags and macros that webwork2 uses for hardcopy production
• tex packags and macros that a particular theme needs

This PR moves everything of the first flavor over to the pg repo. Now in theory something other than webwork2 can fully process individual problem files via tex to PDF, without needing something from the webwork2 repo.

[Alex-Jordan]

I was wondering if it makes sense to move the contents of PGML.tex and CAPA.tex into pg.sty to make sure that file is closer to self-contained.

[drgrice1]

This looks good (paired with openwebwork/webwork2#2099 of course).