TEST: add (modified) rst2ipynb math test cases

[mmcky]

This PR adds various tests for math in rst to ipynb conversion

Display Math

Currently display formulaes are implemented using html for alignment.

<table width=100%><tr style='background-color: #FFFFFF !important;'><td width=75%>$$
x^3+\frac{1+\sqrt{2}}{\pi}
$$

</td><td width=25% style='text-align:center !important;'></td></tr></table>

this should probably be implemented simply as

$$
x^3+\frac{1+\sqrt{2}}{\pi}
$$

• [x] add expected output notebook once defined.

[mmcky]

This PR modifies the implementation to switch to use simpler $$ display math environments in the outputted markdown.