Improve BC

[gouarin]

When we add a boundary condition using for example

auto u = samurai::make_field<double, 4>("u", mesh);
samurai::make_bc<samurai::dirichlet>(u, 0., 0., 0., 0.);

we defined the boundary condition for all components of the field.

But sometimes, we don't want to split our field and we want to have several boundary conditions depending of the field component.

The BCclass must then be updated to take this case into account.