use Crank-Nilson method for differentiating time

- NEW : use Crank-Nilson method to differentiate the continuous equation
utilize the Crank-Nilson method with two method for discretization of the boundary equations

- FIX : fix a bug of the Crank - Nilson method.  problem of the sign
- NEW : add different method for the discretization of boundary vertex with certain boundary condition
- ENH : one can set the boundary discretization method.
- ENH : the boundary condition value is considered to be along the normal direction of the specified vertex.
- ENH : the method of building coefficient matrix and the rhs vector has been enhanced. The situation of bc and inner vertex are treated with different method.
use Crank-Nilson method for differentiating time, not working

- NEW : use Crank-Nilson method to differentiate the continuous equation
Merge branch 'master' into BCMethodTest

Conflicts:
	DDSolver.cpp
Merge branch 'BCMethodTest'
Merge branch 'master' of https://github.com/lunzhy/SimCTM