rename `TBIL.typeset_trigpowers` et al

[StevenClontz]

I like the solution + implementation, but I'm hanging on the naming of variables a bit. E.g. I'd expect TBIL.typeset_trigpowers to return a string or LatexExpr. Since I think you have another PR depending on this, I'll do the renaming in a separate PR.

Originally posted by @StevenClontz in #829 (comment)

[siwelwerd]

While we are renaming things, the following function

library/source/common/sagemath/library.sage

Line 241 in fade7a6

def typeset_angle(theta,latex_delimiters=False):

could use a better name, as it is also useful for getting things like $\dfrac{1}{2}\sqrt{2}$ typeset as $\dfrac{\sqrt{2}}{2}$. But I didn't realize this when I first wrote it (the original intent was to transform $\dfrac{1}{2}\pi$ into $\dfrac{\pi}{2}$, but happily it generalized).

Note that it does return a string, with an optional parameter for including LaTeX delimiters. I wonder if typeset_trigpowers (however renamed) should optionally return a string, and optionally include LaTeX delimiters.