voxel_grid: allow resizing
distance_field: fix bug in negative propogation

negative_update_direction_ was not being set in some situations.  This caused
erronious negative distances inside the field.  It looks like this used to
cause segfaults and was worked around by ignoring cells with invalid
negative_update_direction_.
distance_field: make loops more robust and avoid copies
voxel_grid: make resolution a scalar

resolution is always the same in all dimensions.  No need for an array.
voxel_grid: use multiplication rather than division
voxel_grid: reduce operations function used in inner loops
Merge pull request #73 from acornacorn/groovy-devel

Fix bug in distance field calculation