Refactor CNV frequencies functions to their own module (#665)

* Moving cnv frequencies functions to their own file

* Changes that should help with legacy tests

* Stuck on creating tests.

* Adding some tests for CNV freqs

* Made the same change as in #668

* Added check_type

* Added a test for max_coverage_variance in the advanced version

* Added a test for max_coverage_variance for the normal version

* Update malariagen_data/anoph/cnv_frq.py

Co-authored-by: Alistair Miles <alimanfoo@googlemail.com>

* Removes the old functions in anopheles.py

* Linting

* Updated some legacy tests

* Updated doc, I think

* Added more tests and a nobs_mode parameter

* Apply suggestions from code review

* Finished the merge

* Removed an unused function in test_ag3.py

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Co-authored-by: Alistair Miles <alimanfoo@googlemail.com>

Author: jonbrenas