Fix 'make test' for python 2.6, extend netcdf support.

This fixes make test for python 2.6 that doesn't do "python -munittest discover", by running all tests indivually.
Extend netcdf python support to Scientific.IO.NetCDF and netCDF4. These are different implementation of
(more or less) the same API. It now tries to import from netCDF4 first, then Scientific and then scipy (keep
hearing rumours that scipy's implementation is a bit shit).