Parse MODFLOW6 input (partially)

This is somewhat of a grab-bag of features required for the TKI project.

The primary feature is that it allows reading MODFLOW6 input via imod.mf6.Modflow6Simulation.open. It also includes a number of methods to convert DIS input to DISU input. The DISU package currently has the form of a LowLevelUnstructuredDiscretization: low level, since ideally we can store the information in a UGRID 3D kind of class. Finally, it introduces a StorageBase class which stores the common logic for the two classes rather than having it duplicated.

Fix #138 (closed)