Galion reader: appending option

[alk54079]

Hello and thank you for this amazing library.
I have already used it on some of our Galion .scn files. The only issue that I see here is the following:
The current Galion reader is not capable of iterating over multiple .scn files and merging them into one netCDF. 'Appending' is given as an attribute to the read_to definition, but not really implemented in the coding. It would be nice to add this piece of coding as in the other readers (Windscanner).