Feature added: Get WSI at mpp (#7574)

Fixes: #4980 

### Description

In this pull request, the feature to retrieve a whole slide image at a
given mpp (microns per pixel) resolution was implemented for every
WSIReader class in the function `get_wsi_at_mpp`.

While the implementations in the `OpenslideWSIReader` and
`CuCIMWSIReader` classes were tested thoroughly, I could not find a
suitable TIFF file for testing with the `TiffFileWSIReader` class.

For resizing, I have used `PIL.Image.resize` for Openslide and TiffFile,
and `cucim.sklearn.transform.resize` for CuCIM. Originally, I used
`cv2.resize`, but since the package isn't listed in
requirements-dev.txt, I explored alternative solutions."

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [x] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [x] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [x] Documentation updated, tested `make html` command in the `docs/`
folder.


Please let me know what you think and how I can improve this feature.

Best
Niko

(Last PR was closed, because I changed the branch name to include the
ticket id)

---------

Signed-off-by: Nikolas Schmitz <nikolas.schmitz@rwth-aachen.de>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: NikolasSchmitz