BUG: Add `direct_injection` kwarg to `GlitchWaveformGenerator`

In cases where one calls inject_signal is called directly, the logic we have switched to requires the glitch mode to be labelled {ifo}.

However, we still need the labelling during the likelihood computation, in order to track the correct parameters to inject. So, this kwarg is added to differentiate whether the waveform generator output will be used directly, or applied manipulated before use (as in calculate_snr)