Support for sympy expressions for observables and sigmas

[dweindl]

amici.sbml_import.SbmlImporter.sbml2amici currently expects expressions for observables/sigmas/... as strings. I think it would be preferable to (additionally) allow sympy expressions there, because it will resolve some potential ambiguities of expression strings.

Although not officially supported, it's already possible to do that. However, one has to make sure that the symbols used there have matching assumptions, otherwise this will easily result in hard-to-find model errors (often, only sensitivity equations will be affected). Ideally, for all free symbols, we would check that there is either no symbol with the same name in the model, or a symbol with the same name and same assumptions.