Calculate frame weights

[anotherjoshsmith]

Added reweighting methods for PBMetaD and MetaD. This PR should close #2. Torrie-Valleau can now be performed directly from HILLS and COLVAR files, without using mdrun rerun in GROMACS to get a quasi-static bias. I haven't really optimized these methods, but doing this in python is already way faster than using GROMACS (~1 min vs. a couple hours for a large PBMetaD with multiple walkers project).

[coveralls]

Pull Request Test Coverage Report for Build 36

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• 152 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-27.4%) to 50.445%

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Files with Coverage Reduction	New Missed Lines	%
/home/travis/build/UWPRG/plumitas/plumitas/core.py	152	22.64%

Totals
Change from base Build 34:	-27.4%
Covered Lines:	170
Relevant Lines:	337

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💛 - Coveralls

[anotherjoshsmith]

Planning to make sweeping changes to organization and testing on my next PR. Accepting drop in coverage for the time being, since the changes will invalidate all of the current tests.