Modified THcAerogel::ReadDatabase
1) Need to add fNelem to DBRequest to read parameter arrays.
2) Added optional variable and set to true. Flag in the
DBRequest to tell code that parameter is optional
and does not have to be found.
3) Move setting of fNelem to before DBRequest for aero_num_pairs
 so that it can be overwritten by parameter file.
Merge pull request #26 from MarkKJones/mkj

Modified THcAerogel::ReadDatabase