Allow datetime-like values for LinearInterpolator.
Ensure dates-as-nums are floats.
initial pp constraint
Merge pull request #1240 from marqh/pp_constraint

PP constraint
Update whatsnew v171
Removed spurious line from whatsnew
Merge pull request #1246 from dkillick/update_whatsnew_v1-7-1

Update whatsnew doc for v1.7.1
Bump version to 1.7.1-rc.1
Bump version to 1.7.1-rc.1.DEV
Factored all other PP load rules out of convert()
Document the new datetime capability.
Update missing reference in Cube.regrid().
Minor tweak to user guide.
Remove obsolete image results
Merge pull request #1261 from rhattersley/obsolete-images

Remove obsolete image results
Allow for variance in netCDF4 behaviour.
Merge pull request #1260 from rhattersley/netcdf-masking

Allow for variance in netCDF4 behaviour.
Switch to miniconda test environment.
Update image results for later freetype.
Move cfchecker into conda env