Is your feature request related to a problem? Please describe.
Suite2p is great, but there are a few things that we wish Suite2p could do better. One of them is saving a meanImg in either a PNG or a TIFF file so that we could easily take a look at the registration performance without launching the suite2p GUI. Saving meanImg separately is also useful to train the Cellpose model afterwards. #22

Describe the solution you'd like
We can edit run_suite2p() to accept a new ops dict suite2p_posthoc_ops like below, and make it possible to save meanImg if save_meanimg is true.

suite2p_posthoc_ops:
  save_meanimg: true

Alternatively, we can make a new independent smk postprocess.smk. Another option is to modify https://github.com/neuroinformatics-unit/suite2p to always save meanImg. What do you think? @lauraporta