Store symbolic model during model import

[dweindl]

Store the DEModel during import.

We will need (parts of) the symbolic model for PEtab v2 import (#2662) to update the model state after simulation condition changes. Specifically, we need to know which states are defined in terms of amounts or concentrations and the respective compartment volumes.

This could also be recomputed, but storing the symbolic model seems more convenient for now.

EDIT: alternatively, we need a Model function to compute the updated state that does not process SBML initial assignments.