String formatting with eV? about units

[phlptp]

I will take a look, I am looking at some issues like this

[SimonHeybrock]

Fixed by #130 (thanks!), but I have a related question: In fields where eV is used for energy, there is a set of related units than contain additional odd factors, in particular c (speed of light). See for example table on top right here: https://en.wikipedia.org/wiki/Electronvolt.

The most relevant ones are eV/c**2 for mass and eV/c for momentum.

Would you consider adding support for this? If so, I could give it a try myself (maybe you have some pointers on what is the best way?).

[phlptp]

My guess is adding c, c^2 and eV and possibly h and hbar to the test_units structure on line 319 of units.cpp might deal with a number of cases like that pretty easily.