Detect and remove duplicate dihedrals with the same periodicity

[LSchwiebert]

Resolves issue #529 by issuing a warning when dihedrals with matching periodicities are found. The duplicates are removed and a warning is generated. The warning looks like this:

Warning: Skipping duplicate periodicity of 3 for dihedral HCCTCTHC!

[LSchwiebert]

Updated patch to generate an error and exit, instead of a warning, if the duplicate dihedrals have different parameter values.

[bc118]

Reviewed and it looks good. My only suggested change is some wording. Change the below "Using first entry." to "found duplicate entry" or similar, as it errors out if the entries are different, other than the n-value, which the n-value has to be allowed to be different in this periodic form because it is listed as n entries.

Reading DIHEDRALS parameters. Warning: Ignoring duplicate periodicity of 3 for dihedral HCCTCTHC in GOMC_pdb_psf_ff_files_dihedrals_zeroed.inp. Using first entry.

[LSchwiebert]

Thanks for the feedback. I have revised the error messages per your suggestions.