Check functions now can only return serializable data, and 'ex' and 'got' weren't used later anyway.

symmath_check for simple symbolic input strings.

the expected result = 0.  symmath_check() was dividing
a quantity by the expected result, leading to a NaN value
that caused the answer to be marked incorrect.

Added test_symmath_check.py with basic tests for
the symmath_check() function.

evaluated bool(ans), which is False when ans=0.
This led to an error message "[symmath_check] failed to get MathML for
input; dynamath=[None]" when the function reached a later condition.
The check that evaluated  bool(ans) was redundant, so I removed it.

Make type check for self.code a little more robust, give default val for mmlans so it's never undefined

use new capa_alert div for error messages, clean up debugging

cleaned up some cruft prints -> logging

symmath library; add options for matrices, qubit to symmath_check

main.html now includes mathjax_include for all mathjax stuff
problem.js refreshes mathjax formulas on change
all previous *_fromjs renamed to *_dynamath, eg textline_dynamath