Downgrade netCDF, PyKE and scipy to optional dependencies

A step towards a "core" version of Iris as [discussed](https://groups.google.com/forum/#!topic/scitools-iris/LNpwljIbM8s) with Rich Signell.

NB. While GRIB was already an optional dependency, the registration within `iris.fileformats` has been updated to match that of netCDF. This also removes the need for `iris.fileformats.grib` to use `iris.proxy`.