Draft: Pkgbase refactoring

Split classes in imod.mf6.pkgbase into separate low-level and high-level classes

Fix: #271 (closed)

Steps:

• 1. PackageBase: Copy paste relevant functions from existing Package to low-level and high-level packages.
• 2. PackageBase: Disentangle high-level and low-level data dependencies (e.g. ensure low-level class does not look for "x" and "y" keys in dimensions)
• 3. Create prototype low-level NodeProperty flow package
• 4. Create prototype high-level NodeProperty flow package
• 5. Replace existing NodeProperty flow package with high-level NodePropertyFlow package
• 6. Create prototype low-level Drain package
• 7. Create prototype high-level Drain package
• 8. Replace existing Drain package with high-level Drain package
• 9. Roll out to other packages