mesh manipulation and design

Mesh and MeshData classes should be rewritten or at least reworked

• Mesh::mutableXr() should be removed. The idea is that a mesh should be a constant object. If nodes are displaces (i.e. Lagrange+update), then one should use a new mesh.
• The relationship between Mesh and MeshData should be rethink. May be one could use some kind of repository to store their interactions