remove bundled fonts

[exaexa]

This might be a pretty drastic change for some setups but I believe it's generally for the good of it;

in particular:

• most setups capable of displaying or drawing graphics will have the base fonts available
• bundling helvetica.ttf from mscorefonts is questionable and I'd say Julia registries might object

Additionally this adds an early assert that catches the NaN values (which may come from defunct fonts as I unfortunately observed in #66 ) early and provides a somewhat useful error message.

If the plain removal of the font bundle is too much, I think we can bundle a different one with clear license terms.

[davibarreira]

I did not explain in the issue, but the envelope with NaN is not actually an error. If a diagram is empty, its envelope should be NaN. Similarly, if a text is the empty string "". If no font is found, we should try to use the one provided in the assets.

[exaexa]

Ah wait I thought these are handled by Nothing? (As done in envelope(::TextGeom,...) a few lines below the patch.)

(Generally we were taught that NaN should generally not be used to communicate useful values, only errors that require attention.)

[davibarreira]

You are correct. Its Nothing not NaN. I was confusing the two. This NaN should not be appearing anywhere.

[davibarreira]

The asserts looks good! But I'd like to keep a bundled font. Probably the one you suggested.
Perhaps join #68 here and do a single PR?

[exaexa]

OK I'll reorganize a little.

[exaexa]

I moved the assert to #68, I guess these are the changes that should be generally "ok to go". That leaves the bundle alone, and should be hopefully mergeable.

I'll read up on the TexGyre licensing and see how to move forward here. For now let's have this as a draft.