Source Sampling source.F90 tweaks

This PR contains a few edits to the source.F90:

1) Lines 56 and 81 should have been there in the first place (I think I may have accidentally deleted them when reformatting for style).

2) Lines 77-79 are necessary in the case of non-analog sampling. When sampling a biased PDF, if a bin is selected for which the unbiased probability is zero, the sampled particle will be born with a weight of zero. MCNP does not like this, so I have set the weight to 1E-99. The particle will be terminated upon it's first interaction because it is below the weight threshold.