Remove artifact function with explicit timestamps

RemoveArtifactsRecording takes as input list_triggers, which are in frames. On the other hand, ms_before and ms_after are in milliseconds. This is converted to frames using the sampling rate later. As a result, one cannot make use of ms_before and ms_after if one uses explicit timestamps (perhaps because there are missing samples in the recording). @alejoe91 @samuelgarcia what do you think about changing this such that get_times is used to infer the conversion from ms to frames? This means the padding must be computed separately for each trigger, so the performance may take a hit. Alternatively, could make the user specify frames_before and frames_after, which are computed manually before calling remove artifact.