Loading a subregion of a cube

[CosmicElysium]

Currently loading a fits cube loads the full cube. With large cubes, this can be inefficient when the user only wants to look at sub-regions. It should therefore be possible to load only a subregion from the file selection tab of the desktop gui.

An early implementation can just be having x1,x2,y1,y2,z1,z2 parameters in the file selection tab with a tick box to indicate the subcube loading is desired. It would be good to have these parameters in the desktop gui update whenever a region is cropped by the user.

Later we should be able to deal with WCS versions of these parameters.

[lmarIT]

#248 Have a look to this while fixing this issue.