test_distance_field: fix print function
propagation_distance_field: add getNearestCell()
distance_field: fix bug in adding shapes

Use findInternalPointsConvex() instead of determineCollisionPoints() to better sample the points of the shape.
test_distance_field: fix invalid assumptions

gradient can be 0 even inside a shape (e.g. in center of a sphere).  Fix unit tests to avoid this.
propagation_distance_field: make getNearestCell() work with max_dist cells
fix broken tests