allow user to interrupt HiGHS

[svigerske]

Would be nice to have a way to interrupt HiGHS while it is running, e.g., having a callback that is called after each iteration and that returns whether HiGHS should stop gracefully or not.

In an executable (and in the GAMS interface), this could be used to implement a proper handling of SIGINT.

[mckib2]

Is there a callback function interface available?