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.
Future NetCDF variable promotion.
Expose extrapolation mode for bilinear regridding.
ENH: speedup geometry_area_weights
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.
Merge pull request #1160 from rhattersley/bilinear-extrapolate

Expose extrapolation mode for bilinear regridding.
Merge pull request #1152 from bjlittle/merge_with_nan_fill_value

Merge nan fill_value.
Update
Add Cube.regrid()
Merge pull request #1162 from rhattersley/bilinear-regrid

Add Cube.regrid()
ENH: Implemented array structure identification functionality in
preparation for the next phase of structured PP/FF field loading.