Load reduced grids from GRIB

This PR is based on #682 and adds support for loading data defined on reduced grids from GRIB files. It does this by interpolating the reduced grid onto a regular grid, and then passing the GRIB message on to be handled as a regular grid.

This PR allows reading of both reduced latitude/longitude (`regular_ll`) and reduced Gaussian (`regular_gg`) gridded data, from GRIB1 or GRIB2.

The interpolation method has been tested against EMOSLIB* interpolation and the result match almost exactly. The error between the two fields is less than 0.01 %.

##### To-do:

- [x] Update test data reference when SciTools/iris-test-data#21 is merged


*EMOSLIB is ECMWF's field interpolation library