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

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.