Motor Control library used by ETH boards is now compiled in C++

[marcoaccame]

The files of the motor control library used by the ETH boards are now compiled in C++ mode.

This library was originally developed to run in a C environment on the ETH boards (ems etc).
Over the years, some of its .c files have been added C++ model generated code and so they have been compiled in C++ mode.

Now, we need some other files to contain C++ code, so I have moved all the files into C++ compilation.
To allow a correct compilation on the ETH boards where the functions w/ prefix MController_* are ued by both C and C++ modules, I have done a minor change to the .h files. In particular:

• I kept the dual C and C++ linkage only in the file Controller.h which contains the public interface:

  #ifdef __cplusplus
  extern "C" {
  #endif
  
  // code of `Controller.h` in here
  
  #ifdef __cplusplus
  }       // closing brace for extern "C"
  #endif 

• I added in file Controller.h an opaque struct for the MController which is used only via it pointer. ```
• I removed the C linkage from the other .h files (such as Joint.h etc) that are used only in its internal implementation;
• I added in file Controller.h an opaque struct for the MController which is used only via it pointer.

The files that use the library have been kept unchanged. Only the projects of the ems, mc4plus and mc2plus now compile the files AbsEncoder.c etc in C++ mode.

There is no change in code that requires a new version of the binaries or tests.

[valegagge]

LGTM

[ale-git]

👍