a change in Crank-Nilson discretization method. Direct discretization of boundary equation works. UsingCurrent method fails. Found a bug in the method for refreshing coefficient. did not fix it.

- CHG : change the coefficient in Crank-Nilson discretization method
- FIX : fix a wrong error code
fix the bug in building rhs using UsingCurrent discretization method, this method works without Crank-Nilson method

- FIX : previously,  the time-dependent addent is missing when using the method of UsingCurrent in the discretization of boundary vertex
- FIX : previously,  at boundary vertex, the deltaX and daltaY is not calculated correctly. The length in material other than trapping layer should be exclude.
fix a bug in calculating rhs of inner vertex without Crank-Nilson method

- FIX : removing the addend in rhs without Crank-Nilson method
Crank-Nilson method can be switched and two discretization method can be set.

- FIX : fix Crank-Nilson method in the dd solver. This method can be switched on and off.
- CHG : change of name of some method in dd solver
some revision on the mechanism of simulation time step

- CHG : some change in the class for simulation time step
Fix an fatal bug in setting matrix coefficient and rhs value at boundary vertex

- FIX : fix a bug in filling setting matrix coefficient and calculating rhs value at boundary vertex, in terms of the deltaX and deltaY used in discretization of the equation.
- NEW : add a new method for calculating deltaX and deltaY
- NEW : begin to consider the line density in 2D structure. line density = density * control area
- NEW : add a class for number conversion to string
tiny revision on the filename in method for writing vectors
tiny revision

- CHG : correct the spelling of Crank-Nicolson
- NEW : temp snippet for solving dd in multi steps
- NEW : add the method for printing value in Sctm Message class