code: def get_vds(zdata, delta_y_prime, start, end):
      
      vector_diffs = []
      for k in range(len(zdata)-1): # gets diff between two vectors
          vector_diffs.append(numpy.sqrt(sum((numpy.array(zdata[k+1]) - numpy.array(zdata[k]))**2) ))
      last_vector = numpy.linalg.norm(zdata[-1])
      vector_diffs.append(last_vector)
      vds = sum(vector_diffs)
      f_vds = abs(old_div(delta_y_prime, vds)) # fvds varies, because of delta_y_prime, but vds does not.
      vector_diffs_segment = vector_diffs[start:end]
      partial_vds = sum(vector_diffs_segment)
      max_diff = max(vector_diffs_segment)
      GAP_MAX = old_div(max_diff, partial_vds) #
      return {'max_diff': max_diff, 'vector_diffs': vector_diffs, 'specimen_vds': vds,
              'specimen_fvds': f_vds, 'vector_diffs_segment': vector_diffs_segment,
              'partial_vds': partial_vds, 'GAP-MAX': GAP_MAX}
docs: takes zdata array: [[1, 2, 3], [3, 4, 5]],
      delta_y_prime: 1, start value, and end value.  gets vds and f_vds, etc.