Maxwell arrays are always allocated

Memory profiling shows that Maxwell-related arrays are always allocated, even if we run purely electronic simulations. This needs to be fixed.

23             0.481 Mbytes this%e_field(hamiltonian/mxll_elec_coupling.F90:219)
24             0.481 Mbytes this%b_field(hamiltonian/mxll_elec_coupling.F90:220)
25             0.481 Mbytes this%vec_pot(hamiltonian/mxll_elec_coupling.F90:221)