add emacs function to input optional TeX macro set
make LaTeX-input-optional-macro-set interactive

Still not optimal, since `macro-set-name` does not have a default
value. Will learn how to make that happen when I've got time (my guess
is in summer).
new macros in algebra.tex

\op for \oplus, \ot for \otimes. Clearly I'm learning direct sum and
tensor product now.
Merge branch 'experiment'