Redundant/ambiguous information in amici::Model

[dweindl]

Currently, amici::Model::state_ and amici::Model::simulation_parameters_ both contain fixed_parameters and plist. Those values aren't synced. amici::Model::simulation_parameters_ is initialized during construction and never updated. Setters will only update amici::Model::state_.

This is confusing and likely to cause trouble at some point, and thus, should be changed.