Added support for superscripts in variables and fixed bug with normal subscripted variables raised to powers

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