API inconsistency regarding layer coords and MetaSWAP .from_imod5_data methods

[JoerivanEngelen]

#1335 will implement an imod.msw.MetaSwapModel.from_imod5_data method. What is not tackled here is the following:

1. imod.prj.open_projectfile_data returns grids for the CAP package with a layer coordinate
2. This coordinate information can be potentially required for when coupling MetaSwapModels to other layers than layer 1 (when this is implemented)
3. Right now, imod.msw.MetaSwapModel.from_imod5_data drops this information
4. The package.from_imod5_data implementations require the layer coord to be dropped

This makes for inconsistent API: imod.msw.MetaSwapModel.from_imod5_data is OK with the layer coord that is always added by open_projectfile_data, whereas e.g. imod.msw.GridData.from_imod5_data requires it to be dropped.

This makes for an annoying inconsistency in the API, for when users try to manually import individual packages themselves. I don't see a big use-case for it, but it IS unexpected behaviour.

To get something which works is very easy: xarray has an errors: "ignore" argument for its xr.DataArray.isel method. My concern is more a matter of performance: I haven't tested yet what xarray does when it ignores errors: Does it return a copy or a view? If the first, this would mean that potentially a lot of data would be copied, as the same grids are used for different packages, therefore grids would be copied multiple times for each package. Some profiling on the LHM data would be good to do here to test whether this leads to big bottlenecks.