Unify settings interfaces

[markusgft]

On v2.2 @hamzamerzic had reported the following:

The settings files are quite cluttered and confusing right now.

Looking at the example in NLOC_MPC example in ct_optcon there are three types of settings mentioned there:

NLOptConSettings ilqr_settings; NLOptConSettings ilqr_settings_mpc; ct::optcon::mpc_settings mpc_settings;

I think this could be done in a nicer and more systematic way. Maybe implementing methods like

void setFromFile(const std::string& filename, const std::string& ns, bool verbose)

in each class for which we wish to have this functionality.