Fix normals in plane_slicer_raster_planner.cpp

[InigoMoreno]

Fix issue #179 with normals. The changed line was previously changing the normals according to vx, I used something similar to what is used in the waypoint_orientation_modifiers: https://github.com/ros-industrial/noether/blob/e0b5b17c08e545bff471d9878ba43b3bd25ae27b/noether_tpp/src/tool_path_modifiers/waypoint_orientation_modifiers.cpp#L25

[InigoMoreno]

I also fixed #181 in the same PR, as both are related.

[marip8]

Thanks for the contribution