@ehsteve, a composite lightcurve should be a separate lightcurve class, as the tools needed to work with inhomogeneous data would be quite different.

My big concern with this API is that it breaks away from now `Map()` works.  With `Map`, if multiple files are specified, you get a list of maps back, unless you specify a `composite` or a `mapcube` flag.  Here, you *should* get a list of lightcurves back unless you specificy a `concatenate` flag.