Added a set of classes to simulate other physical quantities.

Implemented heat diffusion based on cellular automata.

Implemented heat actuators and sensors based on physic interaction class.
Added an extended world and a physic simulation class to cope with other physical models such as heat or
electro-magnetic field
Added test directory.
Merge https://github.com/plsm/assisi-playground into heat
Old version of world heat.
Improved class WorldHeat: implemented a cache, initialisation based on world characteristics.
Added a test-heat program to debug the heat model.
Moved computation of heat steady state to another class.
Added an abstract class that is used in physical models based in a grid.  World heat class extends this class.
Added some comments.
PhysicInteraction and PhysicSimulation are now inline classes (everything is in header files).
Added a function that converts a heat value to a colour.
Heat is drawn in either a black or white background.
Added option to write a log.
Tentative code to check heat steady state.
Classes PhysicInteraction and PhysicSimulation are now inline.
Pulled Pedro's changes with heat stimulus.
Removed some merge conflict artifacts from Component.h and test/CmakeLists.txt
Moved extensions to new folder extensions.
Tweaked heat model.
Commented setting of flag GLUT_Xmu_LIBRARY.
Merge branch 'heat' of https://github.com/plsm/assisi-playground into plsm-heat
Pulled in Pedro's changes for heat actuator. Made small mods to successfully compile.