Extend support for PEtab v2 / PySB

[dweindl]

Currently, the supported for PEtab v2 experiments with PySB is very limited. So far, we attempt to encode the initial conditions as piecewise expressions. This doesn't work if the model uses a pysb.Parameter that we can't replace by a pysb.Expression.
Stop trying to modify the PySB model directly. Instead implement the necessary changes at the level of the generated DEModel.