pgfmath parse for width,depth,height

[dginev]

Added in the hope of fixing https://github.com/dginev/ar5iv/issues/431 , but it only resolved the first error in that issue.

Minimal example enabled by the PR:

\documentclass{article}
\usepackage{pgf}
\begin{document}

\pgfmathparse{height("Some Lovely Text")} \pgfmathresult

\pgfmathsetmacro\MathAxis{height("$\vcenter{}$")}

\MathAxis

\end{document}