Identifying Lightcurve inputs in HENreadevents, and modifying the user warning.

[Sudarshan-21]

Fixes issue #129

Description: This enhancement ensures that, if user inputs a Lightcurve, it will first identify, it is a lightcurve, and will notify the user about it, further, it will ask the user, whether or not to proceed, as shown in the attached image. Depending on the chosen option, the file reading or aborting the process will be done.

Current Behavior

At the moment, HENreadevents reads a FITS light curve with just a mild warning. It should realize if it is a light curve, and give a stronger warning or just fail.

Enhanced Behavior

PS: The previous PR is closed, since I was having some conflicts with that forked repo.

[kashish2210]

@Sudarshan-21 thanks for your PR this looks good to me but we have take care of NICER datas handling that is going to handle by stingray (I have lefted some comments ). And also include some checks and tests as per the contributing guidelines. @matteobachetti your review required

[Sudarshan-21]

@kashish2210, thank you for the suggestions!! I will review them and try to make the necessary changes.