Tidy imports.
Add `clean_source` command to setup.py.
Merge pull request #1138 from rhattersley/tidy-grib-load-imports

Tidy GRIB loader imports.
Merge pull request #1139 from rhattersley/clean-source

Add `clean_source` command to setup.py.
Move some experimental.regrid_bilinear... tests.
Merge pull request #1144 from rhattersley/bilinear-move-tests

Move some experimental.regrid_bilinear... tests.
Remove no-coord-system constraint.
Simplify the handling of extrapolated values.
Update NaN/mask handling docs.
De-dupe extrapolation mode definitions.
Merge pull request #1147 from rhattersley/bilinear-no-coord-systems

Remove no-coord-system constraint from experimental.regrid_bilinear...()
Merge pull request #1148 from rhattersley/simplify-regrid-bilinear

Simplify the handling of extrapolated values.
Fixed travis build
Merge pull request #1153 from pelson/travis_gdal

Fixed travis build
Fix `nomask` handling and clarify docs.
Merge pull request #1149 from rhattersley/fix-experimental-bilinear-nans

Fix `nomask` handling and clarify docs.
Define pp.UMVERSION helper for pp rules.
Review changes. Base on namedtuple, improve docs, extra tests.
Rename UmVersion and change API for 'unknown' creation.