constraint classes

[ivalab]

It kind of messes up the way optragen is currently done, but it would be good to convert the different constraint types into their own classes.

• the problem introduced is that concatenation can't be done anymore, since constraints are of different classes. Matlab doesn't quite manage classes the way C++ would, as C++ would be able to do this.
• will need to define concatenation differently: give it some thought.

[ivalab]

Still gonna give different classes a shot since I will be adding more exotic stuff for increased flexibility. Hopefully I can figure out a way to do concatenation.