Add time binning and lightcurve spec

[cdeil]

It would be nice if we had a spec for:

• defining time bins as input to analysis tools
• store light-curves.

From discussion here:

To my knowledge there is no VO standard, but there is http://wiki.ivoa.net/bin/view/IVOA/LightCurves. See also:

• http://wiki.ivoa.net/internal/IVOA/InteropOct2015Applications/lightcurves_skoda.pdf
• http://svo.cab.inta-csic.es/files/svo//Public/SVOPapers/talks/2010_maria_LightCurvesTAP.pdf
• http://svo.cab.inta-csic.es/files/svo//Public/SVOPapers/talks/TimeSeries_raulgutierrez.pdf
• http://www.cosadie.eu/twiki/pub/CoSADIE/TechForum2/sci_priorities_allen.pdf