OpenMP support

This PR adds OpenMP support to musly and libmusly, enabled by the USE_OPENMP option in cmake (or ccmake).
If enabled, the musly command line client parallelizes analysis of multiple files (-a), sparse and full similarity matrix computation (-s and -m), and evaluation (-e and -E), and libmusly is compiled with OpenMP support as well (which is relevant for the commercial similarity measures not included with musly).