Feat: Implement a visualisation feature to NiTransforms

[jmarabotto]

Addition of new file vis.py. Calls nitransform's nonlinear DenseFieldTransform class to illustrate corresponding transform objects through:

• Distorted image and grid
• Quiver plot coloured according to Diffusion Scalar map
• Jacobian map

which can be illustrated all together (3x3 grid of plots, showing axial, coronal and sagittal projections for each graph) or individually.

[effigies]

Can you post screenshots?

[jmarabotto]

Can you post screenshots?

Overview (3x3 plot grid): https://output.circle-artifacts.com/output/job/bb500396-f329-4c10-b994-6ac6365bc443/artifacts/0/tmp/tests/artifacts/show_transform.svg NOTE sliders don't work yet but would be a cool interactive tool for updating slices.

Distorted grid lines superimposed over deformed brain density map: https://output.circle-artifacts.com/output/job/bb500396-f329-4c10-b994-6ac6365bc443/artifacts/0/tmp/tests/artifacts/plot_distortion.svg

Quiver plot of the deltas field: https://output.circle-artifacts.com/output/job/bb500396-f329-4c10-b994-6ac6365bc443/artifacts/0/tmp/tests/artifacts/plot_quiverdsm.svg NOTE colours represent the diffusion scalar map of the vector field (red: dominant x-component; green: dominant y-component; blue: dominant z-component; light->dark: weak dominance -> strong dominance)

Jacobian map: https://output.circle-artifacts.com/output/job/bb500396-f329-4c10-b994-6ac6365bc443/artifacts/0/tmp/tests/artifacts/plot_jacobian.svg

Hopefully this sets a good (or useful) base for the implementation of a visualisation module to nitransforms, or at least can act as a good starting point to spring new ideas.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.25850% with 11 lines in your changes missing coverage. Please review.

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Files with missing lines	Patch %	Lines
nitransforms/vis.py	96.56%	4 Missing and 6 partials :warning:
nitransforms/io/fsl.py	0.00%	1 Missing :warning:

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[jmarabotto]

@effigies this is now ready for review! Latest plots can be accessed through the CircleCi workflow, under build_pytest > Artifacts. These shouldn't be too different (if at all) to the links posted above.