Adaptation of the Lightcurve class to allow multiple file downloads

This modification to the LightCurve class allows a list of URIs to be provided to the _download() method, so that a number of files can be retrieved, and concatenated to form a full light curve.

A modification has been made to the GOESLightCurve class to generate a list of fits files to take advantage of this, which allows multiple-day-long light curves to be downloaded.