Support selecting a percent of total flux given a clean mask file

[sjperkins]

Given a clean mask file produced by Sofia 2, [registers] and groups (https://en.wikipedia.org/wiki/Image_registration) a wsclean component list by source ID's in a SoFiA2 Clean Mask file. Components are lexicographically sorted by (total source flux, component flux) and a percentage of components contributing to total flux are selected.

• [x] Tests added / passed

  $ py.test --flake8 -v -s .

  If the flake8 tests fail, the quickest way to correct this is to run autopep8 and then flake8 to fix the remaining issues.

  $ pip install -U autopep8 flake8
  $ autopep8 -r -i .
  $ flake8 .