More measurement units (in, cm, em)

[Michael Wayne Goodman]

In order to accommodate https://bugs.launchpad.net/bugs/341453, we need to specify size in something other than pixels. A great option is the units used by LaTeX, such as cm, in, em, picas, points, etc. There are probably existing functions for converting these units into something meaningful for a pixel-based measurement system, but this should all happen behind the scenes. Ideally, the user would be able to say:

someplot("myplot.eps", ..., width=8cm, height=6cm, ...)

but perhaps more realistically:

someplot("myplot.eps", ..., width="8cm", height="6cm", ...)
or
someplot("myplot.eps", ..., width=8, height=6, units="cm" ...)