Specify number of threads to use

[mhyee]

In #25, we multithread so that each dimension (objective) gets its own thread, which is scheduled by a thread pool.

The thread pool has N threads, where N = min(USER_VALUE, NUMBER_OF_CORES, NUMBER_OF_DIMENSIONS). Right now, USER_VALUE is hard-coded; this should be specified as a command-line option (via MultiObjectiveOptions).

Anything else that is multithreaded should also use this value.

[joseph39]

For Overlapping GIA, it should be min(USER_VALUE, NUMBER_OF_CORES).