Restructuration of the OpenFLUID documentation

(references OpenFLUID/openfluid#101)
Update of foreword page in doc

(references OpenFLUID/openfluid#101)
Switch to version 2.0.0 final
Update of documentation

* Updated simulator development part
* Added TODO tags where documentation needs to be completed or updated
* Updated Doxygen configuration

(references OpenFLUID/openfluid#101)
Update of the simulators development part of the OpenFLUID Manual