Allow only width or height of figures to be specified

[kcrisman]

From comments on #2100 about aspect ratio:

Would it be possible to extend the functionality so that it is possible to specify either only 
width or only height of the final figure? I am thinking of using it in conjunction with 
SageTeX, where it would be natural to ask either for width=\textwidth or 
height=0.5\texthight and have the second dimension determined automatically based on 
the actual plot and aspect ratio.

This seems reasonable, but was bigger than the goals of #2100.

CC: @novoselt @jasongrout

Component: graphics

Keywords: aspect ratio, aspect_ratio, width, height

Issue created by migration from https://trac.sagemath.org/ticket/10633