Reorder helper functions in fonts.py, clarify 'times' vs 'times new roman', and clarify 'usepackage{times}' vs 'usepackage{ptm}'

[libbylbaker]

This PR tries to make the font file more transparent.

• The functions have been reordered so the helper functions, e.g. font families are at the bottom, and paper font functions are at the top
• Font selection functions have been renamed to make clear which fonts they're defining
• Neurips and ICML paper use times (instead of Times New Roman), so this has been corrected

[pnkraemer]

Thanks!

(We discussed most of these changes offline, so there is not much to review here.)

So technically, the new behaviour of iclr() and neurips() is not backwards-compatible because now they implement times instead of times new roman.

Not that many would notice a big difference between these two fonts, of course... :)