plotbase.plot_phased_mag_series: add model option
plotbase.plot_phased_mag_series: add model option

added option of overplotting a model lightcurve on a phased magnitude
series plot.

also updated docstring.

lgbouma/astrobase,waqasbhatti/astrobase,waqasbhatti/astrobase,waqasbhatti/astrobase,lgbouma/astrobase,lgbouma/astrobase