Fix subsampling resume bug

This PR fixes issue #37, where using the subsampling slider and bringing it back to the lowest value would sometimes "lose" some streamlines.

Problem was caused by casting when getting the min and max length of the streamlines.

This also introduces a small test, that is part of some ideas I have to make the Fibernav better, while we wait to have a new version: some const values are defined for the subsampling range, which are then used in the slider creation. This makes it easier to change the values and stay consistent everywhere in the application (for instance, Fibers and FiberGroup), and will also make it easier to customize the build for people with smaller machines, where we could set a default subsampling value that is for example 50%. This would make it easier on their GPU, when loading by Fibers sets.