#119 add getwarnings() function

[Ali Quis]

In V 0.5.11 a getmode() function was added - appreciated so far!

For a cooperative behaviour in multithreading or multiprocessing applications it would be necessary to have access to ALL internal states (so that you can get+push/modify/pop them).

For such purposes it is suggested to add a "getwarnings()" function as counterpart to setwarnings.

(And maybe to consider to extend this principle to other internal states as f.e. edge detection callbacks)