PyGRB injection finder/combiner fixes

[jakeb245]

This PR fixes a bug in pycbc_grb_inj_finder that caused the entire output dataset to be empty. It was a logical error in checking the datasets to merge. This does seem to cut the search and gating sets. These should be copied to the final output at some point.

An exception will also be raised in the event of pycbc_grb_inj_combiner receiving zero valid input files. This is to avoid the injection sets having improper names. We were calling them "NSXXTEST" but they should be called "NSXX30INJ" where the number is the max inclination.

[titodalcanton]

@jakeb245 you should rebase this branch on master to pick up a fix for the failing checks.