add in a two Stix Characters

[eluberoff]

AIR needs to render these with a custom font.
Note: I couldn't find a latex character code for not parallel

[jaltekruse]

On the not finding a latex code, I'm pretty sure we would want to cover this with general work to get various "not" versions of symobls working, as documented here: mathquill#790

Latex has a general pattern of allowing \not to be in front of many symbols that will produce a crossed out version of them, you can even do this with individual letters

For example:
\not a

Which produces this: