attempt to parse display math for proper PTX md/mrow structure when a…

[Alex-Jordan]

…pplicable

In PTX, multi-lined math expressions are structured with md and mrow tags. Or you can just have the usual LaTeX multi-line math constructors inside an me tag. Up until this PR, we have just done the latter for PG display math being converted to PTX. This PR changes things so that if the display math is an align, alignat, or gather environment, it all gets parsed into an md with mrows.

The following code demonstrates various math structures. Only the last three structures are meant to be affected by this PR, because those are what PTX converts md/mrow to. You can try this before the PR using the PG editor with PTX output, and you will see the display math is always wrapped in me. Then with these changes, the last three have md/mrow structure.

BEGIN_PGML
[`\pi`]

[``\pi``]

[```\pi```]

[```
\begin{aligned}
\pi&=3.14\ldots\\
&<\frac{22}{7}
\end{aligned}
```]

[```
\begin{align}
\pi&=3.14\ldots\\
&<\frac{22}{7}
\end{align}
```]

[```
\begin{alignat}{1}\pi&=3.14\ldots\\
&\approx\frac{22}{7}
\end{alignat}
```]

[```
\begin{gather}
\pi=3.14\ldots\\
\approx\frac{22}{7}
\end{gather}
```]

END_PGML

There is an observable (and desirable) difference in PDF output when the PTX source is structured this way. I can elaborate if anyone cares.

[drgrice1]

Looks fine to me. As usual, you are the only one that really knows what is needed here.