Recursive plotting

[Ilya Zakharevich <ilya@math.ohio-state.edu>]

Of course, after I finished debugging a Math::Pari Perl script which
was 

    auto-refining set of points so that 
         they (the points) give a good graph of the given function
	      if used with cubic splines, 

I discovered this RECURSIVE flag for ploth().  ;-)

Couple of notes:

       a) Documentation says that PARI uses min 15 points for
	  graphing, but analysis of the code says that it is 29 points.

	  Since 15 looks like a right value, one need to change 
	  RECUR_NUMPOINTS to 8, so that PARI uses 15 points min.

       b) I think it is not yet late to change the meaning of flag 2
	  on the opposite : if it is set, PARI will not use heuristics
	  to speed up plotting.

	  One should value backward-compatibility, but with plotting
	  precision does not come first.  I still need more than 5
	  mins to plot zeta on the interval of length 3 near
	  0.5+3000i, and with such speed the time becomes a primary
	  factor.

Ilya