Added netcdf load support for transverse Mercator grid mapping and climatology

This PR results from the changes required to load a netcdf file (see https://github.com/SciTools/iris-test-data/pull/4) with a transverse Mercator grid mapping and  a time variable with a climatology attribute.

Ideally I'd have put these two features in separate PRs and that may be the ultimate result, but as it stands I only have a single file with both of these features so I've bundled them together. All feedback is very welcome.