bcMatrix, boundary condition Matrix

% ====== Providing boundary conditions matrix bcm =========
% ======= (system case, e.g. for 2D plane-strain problem, 2 PDEs for 2 dependent variables) =====
%
% A crutial step for solving boundary value problems (BVP) is to define the boundary conditions (BCs).
% In a recent submission,
% "Solve boundary value probelm by command lines," an simple example is
% given for the scalar case (1 PDE for 1 dependent variable)
% subject to Neumann BC only (sub-function "bcNeumann" in that submission).
% I now try to show how to construct the BC
% matrix for system case (Dimession N, i.e. N PDEs for N dependent variables).
% Dirichlet, Neumann, and mixed BCs are considered.
% A more complex example showing how to use this function will be provided
% soon
%
% ===== By Hongxue Cai (h-cai@northwestern.edu) =======