Merge branch 'refs/heads/master' into pjordan-make-gradient-consider-…

…distance

* refs/heads/master:
  Height of max (metoppv#2020)
  Migrate CLI functionality: simple_bias_correction (metoppv#2018)
  BUG: Fix Scheduled Tests Sphinx-Pytest-Coverage (metoppv#2021)
  Plugin discovery (metoppv#2009)
  Adds masked_add to cube combiner (metoppv#2015)
  Apply mask to cube (metoppv#2014)
  Make difference module handle circular cubes (metoppv#2016)

# Conflicts:
#	improver/utilities/spatial.py
#	improver_tests/utilities/spatial/test_DifferenceBetweenAdjacentGridSquares.py