fix error in compute_sediment: the bathymetry is added to the sea

surface elevation (hydro_sol) and NOT to the tracer
the flux is positive for erosion. the flux has to be multiply by -1
before modifying the sediment bottom value

dgpy/scripts/slim.py

@@ -1048,11 +1048,11 @@ class ShallowWaterTracer2d:
         _flux_dof.scale(dt)
         self._solution.axpy(_flux_dof,1)
         _HC = dgpy.dgDofContainer(self._domain._groups, 1)
-        _H = slim_private._sum(self._solution.getFunction(), self._domain._bath_function).functor
+        _H = slim_private._sum(self._hydro_sol, self._domain._bath_function).functor
         _H_dof= dgpy.dgDofContainer(self._domain._groups, 1)
         _H_dof.interpolate(_H)
         _HC.multiply(_flux_dof,_H_dof)
-        self._sediment_bottom.axpy(_HC,1)
+        self._sediment_bottom.axpy(_HC,-1)
   
 class Loop:
     """Temporal solver"""