ISIS3: preserve label accross conversions

[rouault]

when converting between formats, information from the original label is generally lost. This is for example the case when using the gdal_translate utility to convert from ISIS3 to another format like GeoTIFF and finally converting back to ISIS3. Per this task, we will modify the Persistence Auxiliary Mechanism in GDAL (the one that manages .aux.xml sidecar files) so that it preserves the ISIS3 label, and stores it in a section of the .aux.xml file, when converting from ISIS3 to a PAM-enabled format like GeoTIFF. On the write side of the ISIS3 driver, this preserved label will be used to create the target ISIS3 label (the georeferencing information will be taken from the source dataset, e.g GeoTIFF that might have been subset, and not from the initial ISIS3 label)