Pick a filename relative to METEOPATH in the MeteoIO application

When a METEOPATH is set in MeteoIO, it is not possible to select a relative path for a file compared to METEOPATH. So a file in a subdirectory of METEOPATH will set its STATION# to {filename} instead of {relative-path/filename}... In order to make it work properly, it would need a way to express the relationship between METEOPATH and STATION# in the xml file so a relative path could be returned by the file selector.