Document implementation details for Doxygen (e.g. in c++ files)

[nmellado]

In PR #764 , @MathiasPaulin propose to introduce a variable IMPLEMENTATION_DETAILS to hide/show comments in the documentation generated by doxygen. The idea is to have IMPLEMENTATION_DETAILS enabled by default when compiling the documentation locally, but to disable it in the deployed documentation to ease reading for end-users.

What do you think ?

[MathiasPaulin]

Well, to be more precise, we can add in a next PR a cmake option (false by defaut so that the user must ask explicitely for doc details) to enable this so that so that local developer documentation include some implementation details of Radium internal. This demonstrates the way one can document such implementation details.
And, obviously, I agree ^^ with this.