@rhewett I think we do need the concatenation functionality somewhere in the factory call, partly due to the fact that the data providers give back files per day and if you want a month of data you need to concatenate DataFrames.

@wafels I think if the dates overlap a error should be raised, I can not think of a safe way of handling it otherwise.

@ehsteve It could happen if the user accidentally feeds the same file in twice. So it does need to be handled.

@ehsteve The factory should not automatically concatenate if the files are from the same source, what if you feed two file in from LYRA which are 1 year apart? And you want two LCs back?

The problem here is the difference between this factory and the Map factory, where there is no difference between providing a list of arguments or just a lot of arguments, which makes a lot of sense because there is the `composite` and the `cube` flags for selecting what behaviour you want from the concatenation. I think for the sake of uniformity there should be a concatenate flag like @rhewett suggested. 

Firstly, this makes the API uniform which is important. Imagine a user who has worked out Map, uses it a lot and loves how awesome it is. Then decides she needs a lightcurve, she feeds in lots of arguments expecting the same behaviour as map, which is too return a list of LC objects, and instead she only gets one back. This is very inconsistent and we should avoid it.

Secondly, expanding all the arguments given to LC and only concatenating them if the flag is set will cut a good number of lines out of my implementation.

The only thing I can't decide off the top of my head is what to do if the user specified multiple different sources and the concatenate flag, I think the only logical thing is error.