NAME:
  cumsum
 PURPOSE:   (one line only)
  Cumulative sum of a vector
 DESCRIPTION:
  Element i in the output vector equals TOTAL(invec[0:i])
 CATEGORY:
  Mathematical
 CALLING SEQUENCE:
  csum = cumsum(vec)
 INPUTS:
  vec - input vector to sum
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  return value is the cumulative sum of the input vector
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2020/05/19