Reduce model parameter call

[mknaranja]

In order to simplify the model parameter calls, @reneSchm suggested the following:

Is this `this->template get_flat_flow_index` getting a bit too unwieldy? We could hide it using a private member function, something like

template<...>
inline auto get_flat_flow_index(&&...) {
    return this->template get_flat_flow_index<...>(...);
}

We could do something similar for the parameters, e.g.

template<...>
inline auto get_param() {
    return this->parameters.template get<...>();
}

_Originally posted by @reneSchm in https://github.com/SciCompMod/memilio/pull/895#discussion_r1592079170_