Added options for fileformats.netcdf.save

Here is a first draft of new options for saving netCDF files.

See discussion in #860.

I believe all (relevant) tests are passing. I haven't written any new tests yet.

Most of the documentation was adapted directly from python-netcdf4: http://netcdf4-python.googlecode.com/svn/trunk/docs/netCDF4.Dataset-class.html#createVariable