Adding inject_glitch feature and example

[narolaharsh]

The feature could be of use for various glitch-mitigation or effect of glithces on PE related simulations. Any feedback on furhter developement, relevance of the feature for Bilby welcome.

Specifically, adding the following method to the interferometer object. It injects a glitch at a user-specified SNR and user-specified onset time to the interferometer. A glitch waveform generator is not needed; an arbitrary timeseries can be rolled, scaled, and injected to the detector.

def inject_glitch(self, glitch_parameters=None, glitch_time_domain_strain=None,
                      glitch_sample_times=None, glitch_waveform_generator=None):
        """ Inject a glitch into the interferometer data.

        Parameters
        ==========
        glitch_parameters: dict
            Dictionary of glitch parameters.
            Must contain ``onset_time`` (the GPS time at which the glitch peak should occur).
            Must contain ``snr``. The glitch will be rescaled in such a
            way that it's optimal SNR matches to this value.
             Other parameters passed to glitch_waveform_generator.
        glitch_time_domain_strain: numpy.array, optional
        glitch_sample_times: numpy.array
        glitch_waveform_generator: bilby.gw.WaveformGenerator optional
        ...
        """

Also adding an example script in the usual sub-directory.

Comments from Colm on the previous PR (closed due to incorrect source branch).