Improve example notebooks

[jacob-rosenthal]

The example notebooks should be more comprehensive and better organized. For many people, the example notebooks will be their first experience with PathML, so we want them to be really good.

Some ideas:

• Update the example pipeline notebooks to use chunk processing (much more efficient than the current basic example notebook which loads the whole slide into memory)
• Create a notebook that shows examples of every transform
• Create a notebook showing how to use DataModules with Pytorch

[jacob-rosenthal]

Add example notebook for training HoVer-Net on PanNuke dataset as a full, end-to-end example

[jacob-rosenthal]

Added hovernet training example in #76
Added preprocessing pipeline example in #92