fix(Python/SBML) Fix sensitivities for complex sigma expressions
fix(Python/SBML) Fix sensitivities for complex sigma expressions

Previously, sigmas were expected to be only a single parameter.
If any more complex formula was provided, sensitivities would not be computed correctly.

FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI