kernel2D updated and its equivalent version ckernel2D is added
kernel2D updated:faster 400x400 in 4 seconds, comparing to 0.000278 using C++, also mex -I<dir to eigen> and -largeArrayDims is used, therefore user does not need to go into the mexfile to change dir to eigen
kernel2D updated:faster 400x400 in 4 seconds, comparing to 0.000278 using C++, also mex -I<dir to eigen> and -largeArrayDims is used, therefore user does not need to go into the mexfile to change dir to eigen