NAME:
  obsdur
 PURPOSE:
  Calculate estimate of total duration of an exposure with PCCD.
 DESCRIPTION:
 CATEGORY:
  Data Acquisition
 CALLING SEQUENCE:
  ans=obsdur(filter,exptime,colormode,npat)
 INPUTS:
  filter    - Filter for observation (or anchor filter for color mode).
                 This is an integer from 0 to 9.
  exptime   - Exposure time for indicated filter (in seconds).  Must
                 be greater than zero.
  colormode - Character, 'c' means use color sequence, anything else means
                just this filter.
  npat      - Number of exposures (or color patterns).
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  return value is the estimated time to complete the observation (in seconds).

  NOTE: if any of the input parameters are invalid, this function will
    return a negative number.  That is your indication of an error.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Lowell Observatory, 2002/01/27
  2005/02/07, MWB, updated documentation, added parameter validation
  2009/05/21, MWB, modified for NASAcam