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.