Prevent mosaicing of S3A and S3B files
This changes the logic for grouping processes onto different threads.
Instead of being grouped purely based on date e.g. "20200320" they are now grouped based on date & satellite "20200320_S3A".
I have done some testing and this doesn't seem to introduce any breaking changes but I would suggest some testing before merging this onto the main branch.
Cheers
James