change for testing 2D property of dd solver

- CHG : change the method for setting boundary condition current in dd solver
Poisson solver and dd solver can work together in this version

- ENH : enhance the method for setting density boundary conditions in dd solver
- ENH : enhance the class for vector value
- NEW : add the method of writing Poisson result to file
- CHG : change the position of Message object for outputting simulation message
- CHG : change the method for building vertex map in dd test class