Add Grid for fast neighbors lookup

Useful data structure for any algorithm requiring local neighbors, like in the Smoothed-Particle Hydrodynamic model.
Fix gcc errors/warnings
Add saveScene to Runtime
Grid: Rename members (var. & fnct) + test 3d neighbors + update doc + init->ctor

* static_assert on Number::B base within [2..9]
* rename Grid member variables to shorter names
* rename Grid mapping method to simpler names
* rename Grid other methods to simpler names
* rename some parameters name for clarity
* Get rid of the init method, let the ctor handle initialization
* Add a unit test to test the 3d case with 27 neighbors in 27 different cells.
Grid compute neighbors per cell instead of per element
Move the grid to DataStructures
Grid: Make the cell size a nD vector
Fix doxygen warning
Grid neighbors calculation automated with dirty flag.
Grid: Get rid of the m_offsets unused data structure
Grid: fix class Number documentation & variable names
Fix Grid ctor initializer-list + typo
Grid: Eigen-ize some expressions
Grid: update class doxygen documentation
Grid: remove unnecessary 'void' parameter
Grid: Rename mappingXXX methods to mapXXX + fix typos
Grid: fix cpplint
Grid: ctor takes an AABB. No more power of 2 restriction.
Grid: Update ctor doxygen about limitation due to architecture limit
Add error handling