Fix definition of bcs in particle models

[brosaplanella]

Describe the bug
With the current implementation the particle model doesn't conserve mass when the diffusion coefficient is not constant. This is because the diffusivity at the boundary is evaluated as D(pybamm.surf(c)) which is inconsistent with the definition in the divergence pybamm.surf(D(c)).

We basically need to change D(pybamm.surf(c)) into pybamm.surf(D(c)).