Feature/glace 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.

Now one checks that A(i,j) is accessed with i being the correct
ItemId and j being an integral type. This avoids access such as
- A(i,j) with i being an integral type
- A(i,j) with j being an ItemId

Also add bounds checking for the second index `j` (previously the only
bound checking was if (i,j) was in the scope of the data set, checked
by Kokkos). One also check that `i` is a correct ItemId value, it was
not checked in all cases.