Redesign saving/loading

Problem

1. recursive process on filtering attached/detached FOVs requires non-destructive saving
2. currently we have only one output folder, one loading folder for each type of base data
3. FOVs that are iterated 3 times can't start with the same folder as those that are done 2 times.
4. Non-processed and processed files (output figs) blend together and hart to check.
5. 2D/depth data are not fully synchronized after filtering.
   

Solution

1. at the beginning of an iteration, load the tag file from the same place to retrieve iteration number; at the end of every iteration, update the tag file with an increment in the iteration number.
2. all output folders are named with the iteration number, and loading is done with the correct folder.
3. each process has its own tag series, e.g., stitch-blob-1, stitch-small-2, attach-1, broken-2....
   
4. non-processed FOVs does not output fig/img!!!
5. loading process should be stupid, load() doesn't have to know how to compose the folder names.
6. i_round: current round to be saved; i_round_prev = i_round-1: prev round to be loaded.
7.