Update to read decoy-specific pssm files

[CunliangGeng]

To solve issue #105.

If both reference pssm files and decoy-specific pssm files exist, use the decoy-specific ones for the decoy; otherwise use the reference pssm files.

Note: please first merge RP #103 and then this RP.

[coveralls]

Pull Request Test Coverage Report for Build 583

• 5 of 11 (45.45%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 74.84%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
deeprank/features/FeatureClass.py	2	3	66.67%
deeprank/tools/pdb2sql.py	0	1	0.0%
deeprank/features/FullPSSM.py	3	7	42.86%

Files with Coverage Reduction	New Missed Lines	%
deeprank/features/FullPSSM.py	1	73.85%

Totals
Change from base Build 571:	-0.03%
Covered Lines:	2228
Relevant Lines:	2977

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💛 - Coveralls

[NicoRenaud]

Looks good. We should add test for both cases