Don't eliminate parameters that are initial assignment targets (pt2)

Currently, parameters that are targets of initial assignments don't show up as parameters or expressions in the amici model. This is rather not what most users would expect.

Therefore, treat all SBML parameters that are initial assignment targets and whose initial assignment does not evaluate to a number (for those that do, see AMICI-dev#2304) as amici expressions.
Those static expressions will be handled more efficiently after AMICI-dev#2303.

Related to AMICI-dev#2150.

python/sdist/amici/sbml_import.py

@@ -1095,14 +1095,12 @@ def _process_parameters(
                 }
 
         # Parameters that need to be turned into expressions
-        #  so far, this concerns parameters with initial assignments containing rateOf(.)
+        #  so far, this concerns parameters with symbolic initial assignments
         #  (those have been skipped above)
         for par in self.sbml.getListOfParameters():
             if (
                 ia := self._get_element_initial_assignment(par.getId())
-            ) is not None and ia.find(
-                sp.core.function.UndefinedFunction("rateOf")
-            ):
+            ) is not None and not ia.is_Number:
                 self.symbols[SymbolId.EXPRESSION][
                     _get_identifier_symbol(par)
                 ] = {