Fix for lightcurve truncate function. 

A fix for issue #669.

The truncate function in lightcurve now works, and the truncated lightcurve retains the same sub-class as the original one. Also, removed reference to meta.copy() as the function does not appear to exist.

Doesn't address the issue of what a function like truncate should do with the lightcurve header - it still just copies it.