add option to change unitsPerEm of compiled font?

it turns out one can use ~~fontMath~~ ufo2ft's TransformationsFilter to [change the UPEM of a font](https://gist.github.com/anthrotype/62d0bfe1d38b8f11a199bd3b66574bcc).
This could be useful if one wishes to increase (or even decrease) the resolution of the font design units.
E.g. merging one font with another with different unitsPerEm, or designing with a given unitsPerEm but then scale everything up to reduce rounding errors when generating intermediate instances.

I was thinking maybe such thing could become a ufo2ft preProcessor filter. And the compile functions could have an `unitsPerEm=None` option that, when set, would trigger this filter.

It would compare the requested unitsPerEm with the UFO's current unitsPerEm and scale the output font accordingly to fit the desired UPEM.

Is this too niche or would this be something desirable to have?

Start by reading the linked TransformationsFilter example and then inspect ufo2ft's compile functions to understand where a unitsPerEm option and preProcessor filter would fit. Done means the requested UPEM can be compared with the UFO's current value and the compiled output scaled accordingly, including the unchanged-value case.

Written by the indexing model from the issue text.