Adding an example for supporting matrix multiplication with a user dtype

[danielbarter]

There does not seem to be any examples of supporting non elementwise universal functions, like matrix multiplication. Is this currently possible? If so, it might be worth adding an example. I spent a few hours yesterday attempting to add support for matrix multiplication to the mpfdtype, but was not able to get it working. Any thoughts would be greatly appreciated!

[SwayamInSync]

quaddtype now has a umath/matmul.cpp file which implements and registers the np.matmul ufunc