3D single layer DataArrays trip up mf6 pkg write
This doesn't work properly if the array is three-dimensional with size 1 along the layer dimension:
if layer.size == 1:
recarr["row"], recarr["column"] = (np.argwhere(notnull) + 1).transpose()
recarr["layer"] = layer
else:
ilayer, irow, icolumn = np.argwhere(notnull).transpose()
recarr["row"] = irow + 1
recarr["column"] = icolumn + 1
recarr["layer"] = layer[ilayer]