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
First version of the electric and magnetic field actuators.
Revert "First version of the electric and magnetic field actuators."

This reverts commit 18126083c38e90a32d0965df80d54de2c32eb38e.
Added class Component to master branch.
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.