Integration of IIIF downloading into MapReader

[rwood-97]

Summary

This PR integrates IIIF download into MapReader.
Addresses #77

Describe your changes

• Add IIIF downloader class into MapReader with options of saving georeferenced IIIFs as geotiffs or non-georeferenced IIIFs as pngs.

Checklist before assigning a reviewer (update as needed)

• Self-review code
• Ensure submission passes current tests
• Add tests
• Update relevant docs
• Update changelog

Reviewer checklist

Please add anything you want reviewers to specifically focus/comment on.

• Everything looks ok?

[codecov]

Codecov Report

Attention: Patch coverage is 87.63251% with 35 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mapreader/download/iiif_downloader.py	83.64%	26 Missing ⚠️
tests/test_download/test_iiif_downloader.py	92.68%	9 Missing ⚠️

Flag	Coverage Δ
unittests	79.44% <87.63%> (+0.43%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
mapreader/__init__.py	78.78% <100.00%> (+0.66%)	⬆️
setup.py	0.00% <ø> (ø)
tests/test_import.py	100.00% <ø> (ø)
tests/test_download/test_iiif_downloader.py	92.68% <92.68%> (ø)
mapreader/download/iiif_downloader.py	83.64% <83.64%> (ø)

... and 4 files with indirect coverage changes

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[rwood-97]

See also mapreader-examples for worked examples

[kmcdono2]

Some very tiny edits to the text in my comments, otherwise fine!