Math mode should be always enabled for inline numbers in LaTeX

Reasons:

- Negative numbers get a proper "minus" sign (not a dash)
- Old-style figures can be enabled for a document, but usually aren't for math mode (cf. texdoc newtxdoc)
- Special characters such as \infty (cf. #629)
Merge pull request #635 from krlmlr/ensuremath-always

Math mode should be always enabled for inline numbers in LaTeX