Let grdcut support xarray.DataArray as inputs

**Description of the desired feature**

The `grdcut()` wrapper was implemented in PR #492 and was already merged into master. However, it can't accept xarray.DataArray as input, due to a missing feature in GMT 6.0.0.

After GMT 6.1.0 is released, `grdcut()` should be able to take xarray.DataArray as input.

**Are you willing to help implement and maintain this feature?** Yes