Merge loss into zonal layer

Keep adding stuff to the zonal layer instead of using separate layers. This affects the workflow for the calculation of the Integrated Risk Index, because the data is in one single layer, so the user doesn't need to select layers to join for the calculation.

This PR also fixes a bug in the check for the presence of SAGA in the system. Before, a SagaUtils module was called to check that the installation of SAGA was working properly, but the import of SagaUtils was not checked.