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().

Addition of plot_field for FieldData as alternative to quiver plots.
PhaseMap phase plot now has an option to cut outliers (e.g. difference plots).
IO of PhaseMaps now can handle different file extensions in one loading.
Phasemap creator GUI now uses IO loading.
Replaced flatten with ravel (memory efficiency).
Renamed non constant variables to lower case (not UPPER CASE anymore).