Improve fieldmap visualisation in the reports

[chrisgorgo]

• use cold and hot colormap
• show the mm displacement field with color bar and scale
• show outline of the brain for reference

[oesteban]

Do you have any example of what you want to see?

[oesteban]

See #28 (comment) to see how it looks now:

• use cold and hot colormap
• show the mm displacement field with color bar and scale
• show outline of the brain for reference

The displacement field is not shown because this plot shows field inhomogeneity in Hz. To know how that converts into a displacements field, one needs to know the EPI acquisition details (i.e., imaging parameters of the fMRI or dMRI dataset that the correction will apply to).