Add maximum/minimum methods

hello again

Not sure the names of the methods you prefer. Having other suggestions, let me know, and I'll adapt the PR.

When using these functions for matrices the result can be kind of weird at first. I just made it similar to Matlab/octave that a lot of people know. Nevertheless I am happy to give it other semantic you like. For now I just need to compute the maximum value of a vector, which semantic is much more clear.

If/when you accept this PR, it would be nice a release, if it is not asking too much.

Thank you
Alberto