Add default parameters to decrease spatial and time resolution

[FrancoisPgm]

Reduce the number of voxels to display and constrainthe number of time steps to speed up the color change.

[FrancoisPgm]

time resolution: done
space resolution: currently just select one every n voxel on each dimension, it would be better to use the mean value or a more elaborate dowsampling method

[pbellec]

From the quick demo you had shared with me, it also looked like you were filling the whole field of view with markers. Would it be an option to only show the grey matter?

[FrancoisPgm]

I didn't think about that. On a suggestion of @benjamindeleener I am planning to add an option to select ROIs with checkboxes (given that the user provides the ROI masks). I guess doing the same with grey matter masks wouldn't be much more difficult.

[FrancoisPgm]

Btw @pbellec , would you have a spatial dowsampling strategy to recommand ? Or would you leave that concern to the user and just plot what's provided ?