removed `net_avg` parameter from model call and widget

[jo-mueller]

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Fixes #50

Dear maintainers @carsen-stringer @psobolewskiPhD

This PR

• removes the apparently deprecated keyword argument net_avg from the call to the cellpose model.
• Removes the checkbox from the widget
• Pins the cellpose version to the latest version or newer (>=3.0.7)

I am not entirely sure when exactly net_avg was deprecated but didn't want to go back all the version to find it - hope that's okay :)

[psobolewskiPhD]

I guess the other option is to pin the other way, setting upper bound to the last cellpose 2?

[psobolewskiPhD]

FYI: this has been fixed by @carsen-stringer, the latest release of the plugin works great!