NonLinear Conjugate Gradient method to minimize function f.
Arguments
-  f   : string   : Name of function. Return a real value 
-  df  : string   : Name of f’s derivative. Returns a (R*C) x 1 vector 
-  args: cell     : Arguments passed to f.
 
-  ctl   : 5-vec    : (Optional) Control variables, described below
Returned values
-  x0    : matrix   : Local minimum of f
-  v     : real     : Value of f in x0
-  nev   : 1 x 2    : Number of evaluations of f and of df
Control Variables
-  ctl(1)       : 1 or 2 : Select stopping criterion amongst :
-  ctl(1)==0    : Default value
-  ctl(1)==1    : Stopping criterion : Stop search when value doesn’t
improve, as tested by ctl(2) > Deltaf/max(|f(x)|,1) 
where Deltaf is the decrease in f observed in the last iteration
(each iteration consists R*C line searches).
-  ctl(1)==2    : Stopping criterion : Stop search when updates are small,
as tested by ctl(2) > max { dx(i)/max(|x(i)|,1) | i in 1..N }
where  dx is the change in the x that occured in the last iteration.
-  ctl(2)       : Threshold used in stopping tests.           Default=10*eps
-  ctl(2)==0    : Default value
-  ctl(3)       : Position of the minimized argument in args  Default=1
-  ctl(3)==0    : Default value
-  ctl(4)       : Maximum number of function evaluations      Default=inf
-  ctl(4)==0    : Default value
-  ctl(5)       : Type of optimization:
-  ctl(5)==1    : "Fletcher-Reves" method
-  ctl(5)==2    : "Polak-Ribiere" (Default)
-  ctl(5)==3    : "Hestenes-Stiefel" method
ctl may have length smaller than 4. Default values will be used if ctl is
not passed or if nan values are given.
Example:
function r=df( l )  b=[1;0;-1]; r = -( 2*l{1} - 2*b + rand(size(l{1}))); endfunction 
function r=ff( l )  b=[1;0;-1]; r = (l{1}-b)’ * (l{1}-b); endfunction 
ll = { [10; 2; 3] }; 
ctl(5) = 3; 
[x0,v,nev]=cg_min( "ff", "df", ll, ctl ) 
Comment:  In general, BFGS method seems to be better performin in many cases but requires more computation per iteration
See also http://en.wikipedia.org/wiki/Nonlinear_conjugate_gradient.
See also: bfgsmin.