Add method to display pmf as LaTeX compiled PDF.
Finish dit.utils.latexarray.to_pdf.

Currently, not importing into dit.utils. This is mostly just a toy
example demonstrating some of the context managers and default opener.
Add ExitStack since 2.6 doesn't support "with A() as B, C() as D: pass".
Tab fix.
Merge pull request #46 from chebee7i/pmf2pdf

ExitStack and function to display a pmf in a LaTeX array.