6. In all `*.cpp` files, their include of their analog `*.hpp` file has been moved to the first line of those `*.cpp` files to slightly speed up compiling time.
7. String is used in `include/openpose/wrapper/` to avoid std::string to cause errors for using diferent std DLLs.
8. Added `ScaleMode::ZeroToOneFixedAspect` and `ScaleMode::PlusMinusOneFixedAspect`. Compared to `ZeroToOne` and `PlusMinusOne`, the new ones also preserve the aspect ratio of each axis.
9. Added more verbose to wrapper when it is been configured, showing the values of some of its parameters.
2. Functions or parameters renamed:
1. All headers moved into `openpose_private`, all 3rd-party library calls in headers, and std::string calls in `include/openpose/wrapper/`.
2. Renamed `dLog()` as `opLogIfDebug()`, `log()` as `opLog()`, `check()` as `checkBool()`, and also renamed all the `checkX()` functions in `include/openpose/utilities/check.hpp`. This avoids compiling crashes when exporting OpenPose to other projects which contain other 3rd-party libraries that define functions with the same popular names with `#define`.
