Add a function(s) to filter NuSTAR event lists and produce final event lists

[DavidT3]

This will largely be stage 2 of the nupipeline I think - we have all the spacecraft files we need, and the supporting hot/bad pixel files for each instrument, now we want to take the unfiltered, but calibrated, event lists and turn them into something more suitable for scientific use. This will involve searching for flares and removing bad time intervals (including times where the earth gets in the way, etc.).