Move all shifts into a single file

[ryanjgallagher]

I don't think that it's useful to distinguish anymore between relative and symmetric shifts, and it's more sensible to move them all into a file shift.py. It'd be great if someone could just do from shifterator import shifts as sh or from shifterator import *. Things that need to be done:

• Move them to a single file
• Make sure cross file imports and package loading are done correctly
• Rename anything that refers to "reference" or "comparison"
• Get rid of making the default reference value 0 for the entropy shifts. This is OK because they default to the less detailed shift graphs now
• Decide whether to leave the general shift object by itself and write a wrapper around it in the new file or put everything in one file. Everything in a single file is a lot.
• Get rid of system_1 language, just use type2val for parameters.
• Rewrite documentation and README

[ryanjgallagher]

Might be good to leave shift in its own file if you intend to have weighted average shifts and non-weighted averages (like rank divergence).