Bunch of small fixes, managing agent on EventMap

* include/AreaCell.hpp
isOccupied() changed to const.

* include/EventArea.hpp
* src/EventArea.cpp
Appropriate return types in getters, a bit of comments for managing
agents on map.

* include/sim_crowd/World.hpp
* src/sim_crowd/World.cpp
* src/CrowdSimulation.cpp
Changed field name from 'area' to 'area_'.