def indices_within_voxel_space(i, j, k)
      if [i, j, k].min >= 0
        if i < @vs.nx && j < @vs.nz && k < @vs.nz
          true
        else
          false
        end
      else
        false
      end
    end
def indices_within_voxel_space(i, j, k)
      if [i, j, k].min >= 0
        if i < @vs.nx && j < @vs.nz && k < @vs.nz
          true
        else
          false
        end
      else
        false
      end
    end
Checks whether the given voxel indices describe an index
 that is within the associated voxel space.

 @param [Integer] i the first volume index (column)
 @param [Integer] j the second volume index (row)
 @param [Integer] k the third volume index (slice)
 @return [Boolean] true if within, and false if not