Suggestion: Add a global variable for converting MeV to Joules to be used by all applications

[MartinMartinov]

I'm writing my own usercode right now, and I think that if the egs++ code had a global definition for MeV to J conversion (similar to the global epsilon threshold), it would be easier to ensure consistent scoring results in Gy between codes. It might also be a good idea to split it into two factors:

EGS_Float MeVtoJ = 1.6021773e-10; // Pure energy conversion
EGS_Float MeVtoGy = MeVtoJ/1000; // Convert energy, and mass more gram to kilogram

As the latter conversion is the factor used in most of the codes I've used and would be easier to plug into egs_chamber and egs_brachy, for example.

[mainegra]

I love this suggestion Martin!

[ftessier]

I will integrate this along with #676.