1.1.4 `gnuplot.heat'
--------------------

gnuplot.heat(DT,STEP)
     ::  It solves the heat equation numerical and plots solutions

RETURN
     Void

DT
     floating point number

STEP
     Integer

   * It solves the heat equation  du/dt = d^2 u/dx^2,  u(t,0) = u(t,1)
     = 0 with the initial condition  u(0,x) = x (0 <= x <= 0.5), u(0,x)
     = 1-x ( 0.5 <= x <= 1.0).

   * Heat_N is the number of the meshes in the space.

   * This function will be called `pde_heat_demo' in a future.

Algorithm: NOT Written. (Difference scheme. Courant-Levi-Friedrichs
conditions.)
     [232] Heat_N = 20$
     [233] gnuplot.heat(0.001,30)$

