Implement MRI pixel intensity normalization

[vhosouza]

In MRI visualization there is a big variation in pixel intensity depending on the scanner that the image was acquired. In InVesalius this is not yet accounted for which requires manual adjustments to Window Width and Level for some MRIs that are visualized too dark.

A solution is to apply some normalization techniques before displaying the image and offer the possibility to display the image in the original scale as well.

A non-exhaustive description of normalization methods is presented here: https://medium.com/@susanne.schmid/image-normalization-in-medical-imaging-f586c8526bd1

I suggest using the Percentile scaling which seems to perform well for very different pixel histogram distribution and ranges.