The algorithm field uses four decimal digits for four phases of OT tracking.

  alg = wxyz

****************************************************************
*    1's digit, z: Predict new GS positions from previous positions

  case 0: New = most recent

  case 2-9: Find coefficients among previous observations such that an
            observation is a weighted average of N=2-9 previous.  Apply 
            these coefficients to the N most current observations.
            (N=1 is perforce identical to case 0.)

****************************************************************
*   10's digit, y: Determine guide box offsets from GS positions

  case 0: Take GS prediction at face value, shift box to match

  case 1: Use a median of current GS predictions (and a vote of zero
          shift as well), apply to each

  case 2: Use a median of current and previous 3 GS predictions (and
          a vote of zero shift as well), apply to each

****************************************************************
*  100's digit, x: Choose offsets from science regions from GS positions

  case 0: Use (1/d)^gsexp from guide star as a weight (default gsexp = 2)
          If gxexp < 0, track closest guide star


****************************************************************
* 1000's digit, w: Determine how a PSF centroid is determined

  case 0: Use quadratic fit to FWHM of brightest pixel

  case 1: Use brightest pixel