Reorganize boundary conditions

Now BC types are
- Dirichlet
- Fourier
- Neumann
- Symmetry

One can set tags for Dirichlet, Neumann and Fourier in order to
distinguish variables concerned by these conditions.

In the future, it is planned to allow freefem like conditions such
as `u = g on XMIN` or `alpha * u + dnu (u) = g on 3`,...

However this will only make sense when "algorithms" will be set in
pugs' language.