Creating a file for the code shared by all _frequencies functions

[jonbrenas]

Addresses #664 .

I moved the functions to enable the computation of frequencies and the plotting functions (which required to define a new AnophelesFrequency class). I still want to move the tests of the plotting functions to their own file instead of test_snp_frq.py.

[jonbrenas]

The tests for the plotting functions are still only on SNP frequencies. Is that good enough?

[jonbrenas]

@alimanfoo, I might have found a solution that works for what I wanted to do (namely, import shared testing functions for plotting frequencies into the various test_*_frq.pys) by telling pytest to ignore them when it reaches their file (it means that the tests now return the number of "passed", "skipped", "failed" tests (as well as warnings). I checked that when the tests are actually run when called in the relevant file, they would show a failure if they failed (i.e., they are not skipped when explicitly called).