NAME: 
  fourfunc
 PURPOSE: 
  Compute a Fourier series function (called by fourfit)
 DESCRIPTION:
  This is to be used in cases where you already know the fundamental period.
  The input independent variable is assumed to be reduced to phase of a
  fundamental period already.  The integer part of the number is not
  to the function.
 CATEGORY:
  Mathematical
 CALLING SEQUENCE:
  fourfunc,x,c,f,pder
 INPUTS:
  x - independent variable (phase, between zero and one)
  c - fourier series coefficients
        0 - a(0)    -> constant term
        1 - a(1)    -> cos(x2pi)
        2 - b(1)    -> sin(x2pi)
        3 - a(2)    -> cos(2x2pi)
        4 - b(2)    -> sin(2x2pi)
     and so on for as many elements as in c
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  f    - Evaluated function value
  pder - (Optional) partial derivatives
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  94/10/10, Written by Marc W. Buie, Lowell Observatory