Fixes #4182 .

Description

A MONAI-based implementation of the HoverNet architecture

Status

Work in progress
This is the network + test script, but the full inference pipeline contains post-processing which will be added subsequently.
An example notebook will also be added.

Types of changes

• 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).
• [x ] 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.
• [x ] In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.