mesh support: ugrid, sgrid, SCRIP

As noted in #2 there are several limitations when using NetCDF.

At the moment, we only support structured grids from/to NetCDF files, but there are several conventions to store grids in NetCDF files:

• sgrid: staggered data on structured grids
• ugrid: unstructured grids in netcdf
• SCRIP: simple (un-)structured lat/lon grids defined cellwise