Movable projection plane to highlight a 2D cross-section of the cube

[Pietersielie]

A number of features related to extracting 2 images from the cube remain inaccessible to iDaVIE users. Visualising a single channel can be done by cropping to that channel, but visualising multiple channels in sequence is tedious and extremely time-consuming. Looking at a position-velocity slice is impossible. A movable projection plane in the cube, highlighted by wireframe perhaps, provides a potential solution to this problem. The projection can be displayed in a separate window (like moment maps are already). This plane should be both movable and rotatable, with perhaps a selection of algorithms (specified in config file) used to calculate the non-trivial sampling (e.g., karma's kpvslice algorithm).

See the relevant issues for more information. Development will be tracked by issue #404.