Allow Dirichlet and Neumann boundary conditions simultaneously

[AlePalu]

MESH must allow the specification of boundary regions where the nature of the boundary condition is different. For instance, the user should be able to specify pieces of the border where a Dirichlet boundary condition is set and other pieces where a non-homogeneous Neumann BC is set. The mechanism must be general enought to also support Robin conditions. MESH has to check that there is no boundary point unassigned.