Add a method in Matrix to return a transposed matrix

[Kaiser-Yang]

When we use the lib, we may write a.T() * b which means we want T() to return a transposed matrix but not change the matrix a. But the T may be a bad name.

• transpose(a, output): put the tansposed matrix of a into output
• transpose(a): transpose a in place
• a.transpose(): return the transposed matrix of a without changing matrix a