Default fonts on macos

[tshort]

When compiling the examples, the PDF files look terrible. I assume it's not finding the right fonts.

[breezykermo]

Thanks for filing this bug! Could you confirm what version of MacOS you are running? Rheo is supposed to default to the 'San Francisco' font on Mac, which it seems was introduced in OSX 10.11.

[tshort]

Sequoia 15.6.1.

Font Book doesn't show an SF or San Francisco font. I tried installing the SF Pro fonts, but that didn't help. Also, the SF fonts have pretty restrictive licensing in terms of redistributing documents, so that probably shouldn't be the default.

[breezykermo]

Thanks. The default that I thought I had included was not actually baked into rheo. (It was only set as a default for the rheo.ohrg.org example.) Working on a fix in #77.