Add method get(.), get(.,.) to Matrix and Vector

Using matrix.get(i,j) instead matrix[i][j] is more than useful in terms of performances.
Call to [.][.] create an instance of MatrixRow and then make some operations on it. get(.,.) does not need any instances of MatrixRow to work.

Performances improvements in short:
Measured time for <CPU>: 11.2s -> 5.06s (-65%)
Measured time for <GPU naive>: 1.29s -> 1.13s (-13%)
Measured time for <GPU shared>: 1.24s -> 1.04s (-16%)