housekeeping

Clean core_engine/ (and some in geometry/) very old Bezier-related functions.
They still live on branch bezier.

mv core_engine/ functions for gui-connected sphere world navigation (w, w/o k update) to core_engine/nav_sphere_world.
The remaining in core_engine/ are general multi-trajectory integrators.

mv qtraj_mat2cell.m to matlab plot_utils repo

mv some functions to gui/
rm old k-update dir