Correct math handling, adds Standard_deviation testcase

Adds new test for [[en:Standard_deviation]].

I'm not sure if this is the correct solution, but it fixes the crash
and results in math being substituted in to the text output, e.g.:

"""
$$ 2,\ 4,\ 4,\ 4,\ 5,\ 5,\ 7,\ 9. $$

These eight data points have the mean (average) of 5:

$$ \frac{2 + 4 + 4 + 4 + 5 + 5 + 7 + 9}{8} = 5. $$
"""

Change-Id: I83fef6c2d0db9703a3e60acd69d57a547488c56f