Deuteron production. Resolve LHCBGAUSS-683 ".deuterons"

Closes LHCBGAUSS-683

Code and options file for running deuteron production in Gauss. Choose from two models and set their parameters in the options file.

Considerations to be made for:

• Double counting -- currently takes first binding pair, instead of looping through all possible pairs and choosing the best
• Parents -- don't know if it would be useful to be able to record the kinematics etc. of the parent particles?
• Speed up -- the normalisation parameter for the cross-section model is limited in that the probabilities for a pair binding should not exceed unity. For the maximum allowed value, a deuteron is produced for 20% of events, which is a big increase compared to the nominal value, where it would be closer to 0.1%

Has been tested with a few versions of Gauss (v48r3, v49r9, v50r0), so I think it should work fine for most recent releases.