Fix warnings from compilation

[hlindberg]

This fixes warnings when compiling QuadProgpp.

• There are comparisons between signed and unsigned integers
• A variable 'q' is initialized and set in several places but is never read / used
• There is no checking if delete_constraint finds the given constraint and it looks like it ends up executing some undefined logic if that is the case (it will proceed with a found index of -1 which looks bad). For this problem (since it is no longer possible to set unsigned int to -1) I used an extra boolean flag to make the logic throw an exception if the given constraint was not found.

I manually tested the "main" before and after these changes and the output is identical. There were no other tests to execute.

[liuq]

Thanks for your contribution.