Introduce dummy basins

[TillF]

Is your feature request related to a problem? Please describe.
Virtual / dummy basins can help to establish check-points (e.g. up and downstream of reservoirs). We can have them already, but internally they need meteo data and geo-parameterization, thus, CPU time and storage.

Describe the solution you'd like
Subbasins specified in routing.dat, but without any LUs do not need climate series (or vice versa).

[MelwinQ]

Currently, dummy-basins can be realized by pre-specifying outflow in subbasin_out.dat. In this case, they don't require climate input, nor parameters in response.dat. However, they still need to be listed in soil_vegetation.dat. This should be eliminated.
Potentially flagging a dummy basin (e.g. in hymo.dat?) could also eliminate the need for pre-specifying outflow.