Implement parallel_map for cgpm_metamodel._initialize_cgpm invocation

[fsaad]

If CGPM_Metamodel.multiprocess is True, we should replace the following serial map to use parallel_map:

https://github.com/probcomp/bayeslite/blob/master/src/metamodels/cgpm_metamodel.py#L200-L201

            cgpms = [self._initialize_cgpm(bdb, generator_id, cgpm_ext)
                for _ in xrange(n)]