Remove the Grid class, use Matrix for simplicity.

Redefine the concept of field inheritance :
    Field            -> just a field
    Field2D          -> a field with 2 dimensional space
    Field2D
Remove the Grid class, use Matrix for simplicity.
Redefine the concept of field inheritance :
     Field            -> just a field
     Field2D          -> a field with 2 dimensional space
     Field2DIndex2    -> a 2D field with 2 indices operator(i,j)

Add enumator : Enumerator1, Enumerator2