Improve installation guide and env files for Napari/Qt and PlantSeg

[qin-yu]

Hi @JoOkuma, your software looks awesome! I am a maintainer of PlantSeg and @tischi is trying to use it for one of our project. I had a look at #84 and made a few fixes. It looks like contributions are welcomed, so I'll just make PR here:

• In the environment config in PlantSeg example, I changed the name and channel of PlantSeg, since we moved our software to conda-forge channel already. I also made all dependencies in these files pure conda-based, as they provides the same packages. Specifying a version for pytorch-cuda is recommended by PyTorch and cuCIM installation.
• In the main README and the Q&A in doc, I emphasis the need of installing pyqt via -c conda-forge prior the installation of ultrack, since Napari is a default dependency and it requires but doesn't come with Qt. Note that pyqt from conda-forge channel is the most robust installation from my experience and should guarantee the launch of Napari.

I tested the installation on EMBL cluster and my system to run PlantSeg Napari pipeline and Ultrack tracking on @tischi's dataset without a problem, so I believe it's ready to be merged. Please have a look and see if things need to be modified.

[JoOkuma]

Hi @qin-yu, thanks a lot for the fixes.

The changes don't affect any code run by CI, so I'll merge immediately.

BTW congrats on the go-nuclear pre-print, we will test our data soon.

[JoOkuma]

closes #84