Tau method for linearized problems by newton iterations.

   f = tausysnewton(x, y, ode_system, conditions, varargin)

   f = tauodenewton(varargin) returns the coefficients of approximate
   solution for a linearized problem.

Inputs (required)
   x              = independent tau variable (itau object).
   y              = dependent tau variable (dtau object).
   ode_system     = linearized system of equations (cell of char).
   conditions     = initial or boundary conditions (cell of char).

Inputs (optional)
   exact_solution = exact solution (char).
   step           = step on the x vector to show the results.
   precond        = preconditioner ('no', 'ilu', 'diag').
                    for 'ndiad' define: 'numbd' (number of diagonals);
                    for 'ilu' define: 'milu', 'typeilu', 'droptol', 
                                      'thresh' and 'udiag'.
   solver         = linear system solver.
   apsol          = boolean varargin to show graphically the solution.
   iter           = boolean varargin to show the newton iterations. 
   resid          = boolean varargin to show graphically the error.
   spy            = boolean varargin to spy the T matrix.
   saves          = name varargin to save the results at .mat.
   pieces         = number of pieces if tau piecewise.
   tol            = tolerance for newton iterations.
   maxit          = maximal number of iterations.

   f              = coefficients of approximate solution.