Make data checksum ignore masked values.
Update interpolate.py

fix docstring of `nearest_neighbour_data_value`
Update trajectory.py

fix docstring format in `interpolate` method
Added support for variable resolution fieldsfile loading.
Merge pull request #708 from andreas-h/patch-1

Updated a docstring in interpolate.
Merge pull request #710 from andreas-h/patch-2

Update docstring in trajectory.
Resolved circular imports in Iris.
Merge pull request #619 from marqh/varff

Added variable resolution fieldsfile loading.
Calculates correlation between cubes
Merge pull request #555 from niallrobinson/cube_correlation

Calculates correlation between cubes
BF - Correct Earth radius interpretation for GRIB, fixes #690.
Merge pull request #691 from ajdawson/grib-save-earth-radius

BF - Correct Earth radius when saving GRIB2 files, fixes #690.
Added support for loading regular gaussian grids from GRIB files.
Merge pull request #702 from rhattersley/normalise-checksums

Make data checksum ignore masked values.
Merge pull request #689 from ajdawson/regular_gg-grib

Added support for loading regular gaussian grids from GRIB files.
BF - allow model level GRIB files to be loaded. Closes #720.
Nearest neighbour indexing no longer loads the data.
Merge pull request #721 from ajdawson/hybrid-pressure-grib

BF - allow model level GRIB files to be loaded.
Merge pull request #707 from pelson/nearest_neighbour_no_load

Nearest neighbour indexing no longer loads the data.
implemented y-axis inversion on "positive: down" specified in vertical coord attrs

Replaced incorrect test_1d_positive_attr_down image