Add option to open different orthoview in FSLEYES

[jcohenadad]

When there are more than one contrast, it is extremely useful to have both contrasts opened to generate the manual segmentation. Example below:

To generate this display, see this example, as well as relevant discussion in FSL mailing list.

[valosekj]

Great suggestion! Thank you! I will implement this feature.

[valosekj]

Implemented in #13