Make `sansmath` and `fontset.stix` optional

[pnkraemer]

The font configurations implement sansmath and fontset.stix, which implies that latex-maths is always typeset in the same font as the text, even if the selected font package does not do this.

For example, \usepackage{times} selects Times as the text font and Computer Modern as the maths font. Applying sansmath or stix changes the maths font to match the text font, and therefore, Tueplots' font configuration does not match the journal's font configuration exactly.

In my opinion, this configuration is a clear improvement. However, it is an opinionated font-implementation and should therefore be optional.

The solution would be to expose this choice as a flag in the font-related command. I think the default should be False (opt-in rather than opt-out) because that would imply that the default matches the Latex templates perfectly. But I could be convinced otherwise because it looks much better.