Skip steps within Stitching

## New feature I was trying to stitch a large sample. The whole stitching pipeline seems to have a few consecutive steps, including: - parse czi tile - czi2n5 - retile z - flatfield correction - stitch - fuse where each step could take long to complete. If the pipeline was stopped in the middle, we have to rerun the entire thing from the very beginning. I was wondering if we can skip those steps if they have been done, in much of the same way as we can already choose to skip registration, spot_extraction, etc? Thanks! ## Proposal For example, can we skip czi2n5 if that step has been completed in a previous run? Can we choose to do fuse only, if all previous steps have been completed? ## Usage scenario This will make stitching large sample much easier than now, especially when it is run on a modest server or on a computer cluster that restricts user time.