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.
ENH: Added support for checking netcdf file saving

This commit adds permissions checking and path checking to the netcdf
save within iris.  This tackles the generalised runtime error which
results from netCDF4.