Add netcdf4 package
Add MPI explicitly for netcdf4

This is needed for the hdf5.h, which depends on mpi.h. A better solution would
be to make hdf5 automatically pull in the MPI dependency, but at the moment
even if you put:

run: [mpi]

into hdf5.yaml, the CPPFLAGS/CFLAGS will not be specified for the netcdf4
package, thus the build fails.
Add the python-netcdf4 package
Set library flags for python-netcdf4

This is implemented by adding libflags.yaml/py base functionality.
Build python-netcdf4 in SMC
Merge branch 'master' into netcdf

Conflicts:
	cloud.sagemath.yaml
Merge pull request #20 from certik/netcdf

Add python-netcdf4