Error in Units: electronVolt factor

[duffee]

The factor for eV is given as 1.60E-9 when it should be 1.60E-19

When this change is made, it would be a good opportunity to add in the larger units, keV and MeV for use in particle physics questions and perhaps GeV which gets used for very high energy gamma rays.

[pstaabp]

Fix merged with #690.