Add allocation option for DRN and possibly GHB package.

[JoerivanEngelen]

• _allocate_cells__first_active_to_riv_bot can be renamed to _allocate_cells__first_active_to_elevation,
• which then takes:
  • the bottom elevation for RIV package
  • drain elevation for the DRN package
  • the head for the GHB package.
• In that way the options for the DRN and GHB package can be expanded.

This means the first_active_to_riv_bot__drn can be renamed to something more descriptive. Something like first_active_to_riv_bot__split_at_stage

Also means references to these methods in the example need to be updated.