NAME: 
  colbias
 PURPOSE:
  Determine and subtract column-wise overscan correction with cropping.
 DESCRIPTION:
 CATEGORY:
  CCD data processing
 CALLING SEQUENCE:
  result=colbias(array,x1,x2,xc1,xc2,yc1,yc2)
 INPUTS:
  array - Input array to perform operation on (2-D)
  x1    - First column for overscan region
  x2    - Last column for overscan region
  xc1   - X coordinate of LLHC of sub-array to save (default=0)
  xc2   - X coordinate of URHC of sub-array to save (default=last pixel)
  yc1   - Y coordinate of LLHC of sub-array to save (default=0)
  yc2   - Y coordinate of URHC of sub-array to save (default=last pixel)
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  return value is the overscan corrected, cropped image.
 KEYWORD OUTPUT PARAMETERS:
  BIASVAL - the mean of the overscan area is returned.
  NOISEVAL -the standard deviation of the overscan area is returned.
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  95/06/13 - Initial version written, Marc W. Buie, Lowell Observatory
  2000/02/05, MWB, minor bug fixed when overscan region started at 0.
  2000/11/19, MWB, added support for new data types
  2001/01/22, MWB, changed so bias region is clipped by crop region.
  2006/06/30, Peter L. Collins, Lowell Observatory
              add NOISEVAL output keyword.