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.
Add new extrapolation modes.
Minor tweaks to aid clarity/simplicity.
rewrite example as doctest
Merge pull request #1156 from rhattersley/bilinear-new-modes

Add new extrapolation modes.
Expose extrapolation mode for bilinear regridding.
Merge pull request #1146 from jkettleb/cube_doc

small correction to documentation in iris.cube
Fix (and test) reference surface interpolation.
Merge nan fill_value.