Allow to transport hydrostatic stiffness matrix to an other reduction point

[frongere]

At that moment, the matrix is only expressed at the mesh's origin. We should add a class for this matrix in hydrostatics module. This class has to be quite similar to that used for 3D rotational inertia matrix. For the moment, we do not need to deal with matrix symmetry. A command line option should be added to meshmagick.py