Napari-time-slicer

New features
* Make time_slicer work with functions that don't take a viewer as parameter

Bugfix:
* After conversion, the result stack appeared empty in napari 0.4.13 because the center z-slice was selected per default

New features
* Slice-by-slice annotation for processing 3D stacks using 2D operations

Improvements
* Tests
* Code coverage

Potentially backwards compatibility breaking changes
* `Workflow` and `WorkflowManager` were moved to [napari-workflows](https://github.com/haesleinhuepf/napari-workflows). There are imports for keeping backwards compatibiltiy. Anyway, if you imported those two from `napari_time_slices`, consider switching the import to `napari_workflows`

Bugfix:
* Missing import

Bug fixes
* Removed warning that no viewer was specified