add check to see if IFOs are in Science mode

Work to resolve #131 (closed)

Add a function data_generation.DataGenerationInput._gwpy_data_is_good() to make sure that the data that we get from gwpy is good data.

    @staticmethod
    def _gwpy_data_is_good(start_time, end_time, det):
        """Check if the start-end time is from a period of time when the IFO is locked.

        This method is slow as it queries GWpy.

        Parameters
        ----------
        start_time, end_time: float
            GPS start and end time of required data.
        det: str
            The string key that represents the detector ("H1", "L1", etc)

        Returns
        -------
        True if data is good (IFO is locked during entire duration).
        False if data is bad (IFO is not locked during entire duration).

        """