[Bug] - convert_pwhead_fwhead doesn't work for unstructured grids

**Bug description**
@jdelsman found this. 

There is a check for ``if isinstance(xr.DataArray) and "layer" is pointwaterhead.dims``, 

which should be turned into:

``if isinstance(xr.DataArray, xu.UgridDataArray) and "layer" is pointwaterhead.dims``

I don't think we have any unittests for this function; so it should be considered experimental at the moment.