Running simulations with only field solvers

[sigvaldm]

If we have zero particles and one timestep, with output of field quantities after that time step, it should be possible to run simulations of only the field solvers, with rho=0. With vsources we can add initial conditions potentials to objects, and if we also add an option for setting the initial charge, we can do similar kind of simulations as was done in the PUNC paper. It would perhaps also be relevant to save efield output?