Allow SuperStar calculations to be run in parallel

[Cradoux]

Superstar calculations are the slowest part of the process, so we should give the option to use a pool of processes to run in parallel

[prcurran]

This has been implemented using concurrent.futures

run_ss method now contains the map call. SuperstarResult class has been changed so that the temp working dir is the same for all superstar runs avoiding confusion on where the superstar results are held.

def _run_ss(self, centroid=None):
    """
    initiates a SuperStar run for a given protein and probe

    :param prot: a :class:`ccdc.protein.Protein` instance
    :param out_dir: str, output directory
    :param centroid: tup, coordinates of cavity origin
    :param charged_probes: bool, if True 'positive' and 'negative' probes will be used
    :return: a :class:`SuperstarResult` instance
    """

    if self.charged_probes:
        self.probe_dict = dict(
            apolar='AROMATIC CH CARBON',
            donor='UNCHARGED NH NITROGEN',
            acceptor='CARBONYL OXYGEN',
            positive='CHARGED NH NITROGEN',
            negative='CHLORIDE ANION'
        )
    else:
        self.probe_dict = dict(
            apolar='AROMATIC CH CARBON',
            donor='UNCHARGED NH NITROGEN',
            acceptor='CARBONYL OXYGEN')

    wrk_dir = _test_output_dir()
    args = [(k, self.probe_dict[k], centroid, self.prot, self.out_dir, wrk_dir) for k in self.probe_dict.keys()]
    print(args)

    ex = futures.ThreadPoolExecutor(max_workers=5)
    results = ex.map(self._superstar_job, args)
    return list(results)

This should speed up the calculation time significantly!