Fix for GOES lightcurve range-based queries

This fix addresses issues found in #658, #881 and #890. When constructing a GOES lightcurve based on a time query, slicing was being handled incorrectly, leading to buggy behaviour, e.g.

    lc = lightcurve.GOESLightCurve.create('2013-06-01','2013-06-02')

The date portion of the start and end times was being ignored during the data slicing, leading to strange behaviour.