Add classmethod to construct BoundaryCondition from iMOD5 data

After #961 is fixed, we can expand this method for the BoundaryCondition package to do:

• Test if planar grid included (no layer)
• Allocate cells (for RCH, GHB, RIV, DRN)
• Distribute conductances (for GHB, RIV, DRN)

This can be done in function with something like maybe_allocate_to_layers()

class BoundaryCondition:
...
    @classmethod
    def from_imod5_package(cls, dis_pkg, npf_pkg, **data):
        # target grid is idomain
        target = dis_pkg.dataset["idomain"]
        # Regrid to one consistent dataset
        dataset = cls._regrid_like(target, **data)
        # Allocate to layers, if has planar grids
        dataset_allocated = maybe_allocate_to_layers(dataset, dis_pkg, npf_pkg)
        return cls(**dataset_allocated)
...

Review issue #961 first, then locate the BoundaryCondition package and its proposed from_imod5_package entry point. Trace _regrid_like and the proposed maybe_allocate_to_layers flow using dis_pkg, npf_pkg, and the provided data. Done means planar grids are detected, cells are allocated for RCH, GHB, RIV, and DRN, and conductances are distributed for GHB, RIV, and DRN.