Add new extrapolation modes.

FYI, the follow-up PRs to this should be:
 1. Expose the new extrapolation modes via `regrid_bilinear_rectilinear_src_and_grid`.
 2. Wire it all together:
  - Add `Cube.regrid()` (which will use a new `Linear.regridder()` method to create a `LinearRegridder` which just defers to `regrid_bilinear_rectilinear_src_and_grid`).
  - Modify `iris.analysis.interpolate.regrid()` to use `Cube.regrid()` when in bilinear mode.