new CubeList method to extract over common coord values
Changed to generator comprehension
Default NetCDF dimensionless unit.
Simplify circularity test in nc load rules.
Define circularity from bounds, if any.
Added tests for old+new circular logic.
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
Merge pull request #1184 from pp-mo/nc_bounded_circular

Mark a longitude coordinate loaded from netCDF as "circular" if the bounds look right.
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.