use cv::AutoBuffer::data() to get data pointer

- added .data() methods
- added operator[] (int i)
- extend checks support to generic and debug-only cases
- deprecate existed operator* ()

Not all backends support -1 index.

The result of PyUnicode_AsUTF8() is now of type const char * rather of 
char *.

Add Java and Python code for AKAZE local features matching tutorial. Fix incorrect uses of Mat.mul() in Java code.

Uniform Lowe's ratio test in the code.

`src_pitch` may be large than data `step`