Support parallelism via OpenMP and n_jobs param

[gagolews]

        @TODO once n_jobs params are added
        if c == "msvc":
            for e in self.extensions:
                e.extra_compile_args += "/openmp"
        elif c == "mingw32":
            for e in self.extensions:
                e.extra_compile_args += "-fopenmp"
                e.extra_link_args += "-fopenmp"
        elif c == "unix":
            # Well... gcc/clang has -fopenmp,
            # icc has -openmp, oracle has -xopenmp, etc.
            # The user should specify CXXFLAGS and LDFLAGS herself, I think.
            pass

[gagolews]

exact=False: FAISS uses OpenMP/GPU see https://github.com/facebookresearch/faiss/blob/master/configure

[gagolews]

exact=True:

uses internal.mst_from_distance which is based on openmp'd CDistances moreover, if M>1, then sklearn.neighbors.NearestNeighbors are used to compute d_core first

[gagolews]

n_jobs -- set OMP_NUM_THREADS