develop ini syntax to couple multiple instances from same model (framework)

[DirkEilander]

Right now we can only use the abbreviations of the implemented models (PCR, DFM etc) in the ini-file syntax under the [models] and [exchanges] sections. As wflow is a framework with several models rather than one model there are usecases where we want to couple a wflow hydrological model with the wflow kin wave routing or reservoir models. For these usecases we need to be able to have multiple instances of the same model. A possible solution would be to make it possible (optional) to use the following syntax (or similar) to reference

[models]
W3RA=WFL/W3RA/wflow_W3RA.ini@WFL
routing=WFL/W3RA/wflow_routing.ini@WFL
DFM=DFM/dfm_settings.nam@DFM

[exchanges]
# runoff directly from W3RA Sr0 variable
W3RA.Sr0*cellArea=DFM.rain*ba*1000@1d
# upstream routed discharge from wflow routing module
routing.Qtot*86400@grid_us=DFM.rain*ba*1000@1d_us