Refactor rank filters

* Renamed source files for consistent naming
* Replaced numpy arrays with typed memoryviews
* Overall styling and comment improvements

EDIT:
* Now with full 16bit support
* Partially improved test cases

EDIT2:
* combined implementation for uint8 and uint16
* entropy now returns exact same results for uint8 and uint16
* auto-level now works correctly (previously did not return correct values for uint8)
* all functions support uint8 and uint16 out of the box
* number of bins matches now exactly the maximum value in the image (2**N steps before)