Add modifiers

[markaren]

This PR allows any number of modifiers to be added to a scalar_connection.
A gain_modifier has been implemented.

auto con = std::make_shared<cse::scalar_connection>(out, in);
con ->add_modifer(std::make_shared<cse::gain_modifier>(2));

Happy to receive feedback.

Let me know if this is wanted or not. Perhaps you guys have envisioned another way of achieving the same. Then please share :)

Note: I think modifiers should have access to currentTime and deltaTime. E.g. in order to implement filters and or condition based modifiers.

Resolves #378

[markaren]

Closing as we agree that modifiers are better to have on inputs/outputs