Pack of formal changes when dealing with Mat::data

Several type of formal refactoring when dealing with Mat::data
1. someMatrix.data -> someMatrix.ptr()
2. someMatrix.data + someMatrix.step * lineIndex -> someMatrix.ptr( lineIndex )
3. (SomeType*) someMatrix.data -> someMatrix.ptr<SomeType>()
4. someMatrix.data -> !someMatrix.empty() ( or !someMatrix.data -> someMatrix.empty() ) in logical expressions