Fix reading of renamed files

[Martin Raspaud]

At the moment, the satpy reader for pygac is dependent on the filename format of the gac files. It would be really good to have a function that takes in a filename, and by looking at the header could determine what type of data it is: pod vs klm, gac vs lac.

[Carlos Horn]

Isn't this one solved by the new function runner.get_reader_class in #55 ?

[Martin Raspaud]

I think it does, thanks @carloshorn !