stingray/lightcurve: Validated input time array
stingray/lightcurve: Validated input time array

Code added to issue a warning to the user if the input time
time iterable is irregularly spaced, i.e. the bin sizes aren't
equal throughout.

Solves #77

dhuppenkothen/stingray,abigailStev/stingray,evandromr/stingray,StingraySoftware/stingray,pabell/stingray