Permit to append "GDAL metadata" on the fly

"GDAL metadata" are added only after orthorectification is done.

Yet several metadata are completely unrelated to orthorectofication. They may simply concern the file (polarisation, ortbit...), the calibration, or they may need to be updated later on like the acquisition date once the concatenation as been done.

Ideally, each step (calibration, cutting...) shall be able to register new metadata to add/update and the actual modification would be done after ExecuteAndWriteOutput() has been executed.