Move simple commands to .sty file

[yoachim]

I took a stab at moving the simple AASTeX commands to a .sty file. The use case is for times when you need to use a template .cls file (e.g., the Astro2020 white paper call), but would still like all the usual journal commands to work. Tables and figures might still need to be redone, but this makes it easier to copy-paste blocks of text and references between documents.

Took a pass at updating docs/aassymbols.tex. The \apj command in there is not doing what I expect, but darned if I can figure out where it's getting redefined.