[BugFix]: Truncated lightcurves should remember their sub-class

A one-line PR!

Truncated lightcurves should remember what type of lightcurve they are (e.g. NoRH, LYRA), instead of falling back to the generic lightcurve class.


@aringlis Can you change this to `self.__class__.create`? The class method should be called on the class object rather than an instance of the class.