Input for _flag_times

[JacksonJMB]

Hi @joshoewahp,

I think it would be useful if _flag_times in dynamic_spectrum.py (dev branch) allowed input of Time objects, or to have some other way of specifying the date as well as the time to be flagged. I've got some times that I'd like to flag but at the moment I'd need to use if statements to choose different flag_times in DynamicSpectrum to make sure the flagging is only applied to the relevant epoch.

Thanks :)