Basic support for varying longitude ranges.

This pull request adds better support to `iris.analysis.interpolate.linear` for circular longitude ranges with different starting points.(*)

For example, starting with a Cube defined with longitudes covering the range [-180, 180) it is now possible to re-sample correctly with longitudes in the range [0, 360).

(*) Created in response to a user bug report where `iris.analysis.interpolate.regrid` was failing to deal with two Cubes with different longitude ranges.