Support allocation results in Delft-FEWS

[visr]

This probably has similarities to #2166. To be able to debug allocation results over time and space we probably need support in FEWS.

These are the two result files for allocation:

https://ribasim.org/reference/usage.html#allocation---allocation.arrow
https://ribasim.org/reference/usage.html#allocation-flow---allocation_flow.arrow

Both have the discrete variable priority. Perhaps this can be implemented like substance for concentration, using realizations (ensembles)?

The allocation_flow file also has optimization_type, see #2195. Though those are fixed so perhaps can be split into separate variables.

For QGIS we have #1478

[gijsber]

Best to implement the priority label items as a priority realization (similar to substance realization).
non-priority based items (e.g. the total demand and total realized) should not be in the realization but can be included as a normal scalars (similar to basin.nc or flow.nc)