Class: MatrixGrader

[ChristopherChudzicki]

This does a couple of things:

• minimize global defaults like DEFAULT_FUNCTIONS. Each mathy grading class now has class attributes default_functions, default_variables, and default_suffixes that determine the defaults used by that class.
• Adds new MatrixGrader class. MatrixGrader has some extra default variables/functions, and a few extra configuration options.

Should resolve #71, #76

[jolyonb]

Oh, one more thing - can we make a list containing the Pauli matrices for me, so that I can add them as constants to a problem easily?

Edit - just saw your comment on the issue. Maybe it's better to just leave it out.

[ChristopherChudzicki]

@jolyonb I think this is ready for re-review.

[ChristopherChudzicki]

Oh, I forgot to restore default functions/variables. Doing it now.

[ChristopherChudzicki]

@jolyonb This is actually ready now. I think #89 should be resolved separately.

[jolyonb]

Agreed on #89. Everything else looks good; just holding off on the Pauli nomenclature, but good to merge after that.