Fix netcdf IO

[loerum]

lons.data.fill_value and lats.data.fill_value fail. Instead using local variable fill_value (is that correct ?)

Make MPOP's own nc_reader happy, by joining name and number with a '-'

[coveralls]

Coverage remained the same at 12.196% when pulling 43f686e on fix-netcdf-io into 3ced9c5 on pre-master.

[loerum]

Hmm, my PEP8 checker, complains about line 298 and 303 in cfscene.py where variable off_attr and sca_attr is undefined.