Remove redundancies

[epeters3]

Closes #184. This PR makes these contributions:

• Eliminate the need for metafeatures.json -- now the metadata used for the metafeature/resource computation only needs to be declared once; right where the computation method itself is declared.
• Add facilities for specifying a metafeature's problem type and any associated metafeature groups.
• Get rid of unused profile_distribution first return value.
• Makes the data about the resources and which metafeatures are computed easier to read.
• Factor out the constant strings from the Metafeatures class to a constants.py file, and refactor to always use those constants, rather than string literals sometimes.
• Add a basic CONTRIBUTING.md guilde.

This PR eliminates 2981 lines of code from the codebase (by eliminating the metafeatures.json file).
Note: An overview of what has changed in this PR can be found in ./CONTRIBUTING.md

[epeters3]

Ok @bjschoenfeld and @macetheace96, I think I've addressed all your excellent comments and questions. Looking forward to any additional feedback you have before we merge this PR.