NEW: Add Feature Model Nuclei Instance Segmentation

[vqdang]

• Add HoVerNet Architecture and associated post-processing methods
• Add Nucleus Instance Segmentor to specifically deal with nucleus instance segmentation on WSI levels
• Add metric function to assess detection quality.
• • F1 detection basing matching point sets.
• Add function to overlay instance prediction onto canvas or image.
• Add Tests.

May need another PR to actually improve the doc and better explain the post-processing scheme.

[simongraham]

Nice work. Overall it is looking good - mainly comments to do with docs. Also, please clarify how easy it is to function tiles in addition to wsi ? Ideally we need to be able to allow this with the addition of an argument like we do in patch predictor. Getting the tile level results will be a requirement. If it is incorporated then it needs to be made clear as it seems like it is all geared towards wsi at the moment.