phase out *mathspace mspace values

[dginev]

Fixes #1801 .

Questions to @brucemiller :

• Should we use the em units MathML Core recommends, instead of pt, so that spaces scale along with font-size?
• Is the PR correct so far?

I can claim the current commit phases out the old values, as a grep for "mathspace" doesn't find any other active code with them. But so far this is the kind of change that someone completely new to latexml would made, since I consider myself mostly novice when it comes to the post-processing unit considerations.

Thanks for any tips!

[brucemiller]

looks plausible, and those few mathspace's were only used for internal spacing tweaks, so pretty innocuous allround.

[dginev]

Alright, good to merge on my end if so. You can always revisit them in the other spacing work.