Cellpose

* fix bug in diameter changing when rerunning the model after training -- should make Cellpose 3 training parameters now equivalent to Cellpose 2
* fix bug with nimg_per_epoch being fixed to 8 when training in the GUI (925)

- Update install instructions to include `venv`
- Documentation fixes and additions
- Small bug fixes
- fix `io.masks_flows_to_seg()` iteration bug

Cellpose3 release with CLI, API and GUI support! Check out docs for updates to code: https://cellpose.readthedocs.io/en/latest/restore.html

first Cellpose 3 release!

Last release of Cellpose 2 before update to Cellpose 3.

- fix slow stitching problem by only checking ROIs for stitching in current plane (issue 845)
- fix reshaping issue in flows from mrariden ('mrariden-dynamics_reshape_fix' )
- add python 3.11 support from harryzcy (807)
- disable multiproc on windows from mrariden
- fixes bug in 3D interpolation for multi-ROI masks
- disable gui autosave from mrariden (784)
- fix min size from SimonBon (781)
- fix imagej_roi_converter from francodgstn (828)
- fix torch version command (831)
- fix zaxis=0 bug from nickeener (688)
- add a pyqt version to env from rhoadesScholar (849)
- fix for multimodel adding to gui from landoskape (839)

Changes added to main branch
- Added functionality to allow saving ROIs in ImageJ-native format.
- Minor bug fix related to saving images with many objects thanks to alix-simon
- Performance improvement using improved indexing thanks to sophiamaedler
- Update Read the Docs to include a FAQ section, and misc RTD updates
- Add multithreading to 'outlines' exporting to speed up saving/execution time