Add support for DataArrays #35
Description
Right now our classes expect a Dataset as input.
But if we only need the Bathymetry data and coordinates, we can easily support DataArrays as well.
For example, if da is a DataArray (bathy data) with all {e1,e2,gphi,glam}{t,u,v,f} as coordinates, Zco(da, jpk).__call__(...) will return the same DataArray adding {e3,z3}{T,W} as coordinates.