Task1 refactor epimutations: separate dataframe creation and filtering

[jamesbaye]

• epimutations now generates the regions first, then optionally filters them according to the criteria specified by the epis_kept argument.

• Simplified epimutations input arguments:
  -- Renamed method to stats to reflect that it controls only the outlier score statistics.
  -- Added epis_kept to specify an optional epimutation filtering criteria. Implemented the aref-eshghi criteria.
  -- Removed all the pre-existing filtering arguments.
  -- Added args.stats to optionally pass arguments to the outlier stats methods.

• Added beta_diff column in the epimutations output dataframe: the average regional methylation difference between case and controls.