Kernel corrections and ForwardModel properties enhancement (more modular now).

kernel: u and v are switched and now finally correct.
        get_elementary_phase is also switched and corrected.
phasemapper: added PhaseMapperCharge from Feng Shan.
forward_model: now uses y instead of phase_vec/Se_inv and now direct properties.
costfunction: corrected some docstrings and documentation comments.
fielddata: corrected some missing integer divisions (// instead of /),
        added plot_streamline().