simplify RNGSettings logic for setting the seeds

[anilbey]

Currently, Synapse objects are doing redundant work on checking and assigning RNGSettings values. See if the RNGSettings checking and assigning can be moved to the Simulator.

        if cell.rng_settings is None:
            self.rng_setting = bluecellulab.RNGSettings(
                mode='Compatibility',
                base_seed=base_seed)
        elif base_seed is not None:
            raise Exception('Synapse: base_seed and RNGSettings cant '
                            'be used together')
        else:
            self.rng_settings = cell.rng_settings

[anilbey]

This is fixed. With #149 neither Synapse not Cell store an rng_settings object as an attribute.