Added parameter incidx for plot_lines and bugfixes.
- One can now also choose the inclination for plot_lines if the result of the line ray-tracing is used
- renamed all __incidx to _incidx, do not want the name mangling (is somewhat confusing for this scenario)
- I fixed some places where the inclination or the unit for the lines was not correctly set.
Addresses #38 (closed)