rch and evt (and uzf in PR #263) add variables "rch_name" and "evt_name" to ds

[rubencalje]

ds_to_rch, ds_to_rvt and ds_to_uzf (in gwf.recharge) add variables "rch_name" and "evt_name" to the model Dataset. These DataArrays contain strings with the name of the timeseries that are added to these packages.

I propose to not add these variables to ds anymore, as I do not think they are useful for the user later on.