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

Add Cube.regrid()
Add 'ukmo__um_version' attribute for pp load and save.
Fixed cml and cdl changes.
Fix doctests and examples.
Fixed um_version attribute name.
Revised docs and docstrings for new um_version attribute.
Revised cdl/cml files for new um_version attribute.
Remove version code from netCDF 'source' attribute.
Revised docs and docstrings for 'source' attributes without um version in.