adding C++/matlab example, channel flow / cylinder flow

[yiyuef]

Hi,

I've previously done the matlab version of a channel flow problem (2D N-S equation, incompressible, inviscid), and I've almost done the C++ version. As I mentioned in the previous issue I got some trouble in building the environment, but I've done the code along with those existing functions seperately.

I attach some of my results below. I can also create a PR to uplaod my discription words of my code.

Would this be a good idea?

Thanks!