Add asserts to testing for sw sub-timestepping

[rleander]

Several tests where added to the RibaMetaMod test section with dtsw<dtgw.
There should be 4 of these:

• dtsw = 1, dtgw = 1

• dtsw = 0.5, dtgw = 1

• dtsw = 1, stgw = 2

• stdw = 0.5, dtgw = 2

Results should be compared two by two to assert that the results are invariant to surface water step size halving