NAME:
  initsite
 PURPOSE:   (one line only)
  Initialize a regular grid of occulation station into a file
 DESCRIPTION:
 CATEGORY:
  Occultations
 CALLING SEQUENCE:
  initsite,fn,nsites,spacing
 INPUTS:
  fn      - String with name of file to write to.
  nsites  - Number of observing stations
  spacing - Cross-track spacing between each station, in km.
              If this number is negative, the value is taken to be the
              cross-track spread in km and the spacing is then computed
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  OFFSET  - Offset to the pattern, in km. default is 0 which would be centered
             on the middle of all the sites.
  PREFIX  - Prefix character for site name, default='Z'
 OUTPUTS:
  File is written with the desired information.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
  Limited to 99 sites due to the site code output string.
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2018/10/24
  2019/09/14, MWB, added PREFIX keyword option