Structural modification of the documentation
to make it clearer.

TODO: Homogeneous relationship between debugging information.
Giving more python examples.

ref https://github.com/stack-of-tasks/sot-core/issues/58

    g_pool and g_factory global variables have been replaced by static methods
    getInstance in each class.
    getInstance returns a pointer to the unique instance of the class and
    creates it if needed.

Figures are to be converted to png so that all browsers can read them,
however svg files are be kept to allow futher editions of the figures.

Figures are to be converted to png so that all browsers can read them,
however svg files are be kept to allow futher editions of the figures.

* Moved entity.png to doc/figures so that it can be found by doxygen (cannot use the svg version because it breaks the layout in my browser - Chrome).
* Corrected some doxygen errors.

    * CMakeLists.txt: build doc by default,
    * doc/package.dox.cmake: headers are now in include directory.