Warn user if updating wrong settings after setup

[abrandemuehl]

Some settings can't be updated after calling setup, now users will be warned if they try to do so after setup

[CLAassistant]

All committers have signed the CLA.

[imciner2]

Thanks for doing this and sorry it hasn't been looked at until now. There is actually already a check in the Matlab class wrapper here https://github.com/oxfordcontrol/osqp-matlab/blob/master/osqp.m#L569 that is supposed to stop the setting of these settings after the initial setup. It would be better if the other settings that don't work after initialization are added to that check instead of in the mex file.