Matplotlib issues with "thin" font styles and .ttc font files

[lukelbd]

With Helvetica Neue as the preferred font, the "thin" style can overwrite regular styles on macOS (matplotlib/matplotlib#8788). The next version of proplot will no longer try to use Helvetica Neue -- it will just try to use Helvetica.

[lukelbd]

There is also evidently an issue when trying to save figures with .ttc fonts as PDFs (matplotlib/matplotlib#3135). The next version of proplot will also remove all .ttc font files from font_manager.fontManager.ttflist, and issue a warning when users put .ttc files in their .proplot/fonts folder.

[lukelbd]

I have also added sections to colors_fonts.ipynb that describes how to add your own fonts & add your own colors.

[lukelbd]

Closed by #95