Add Polygon structure in pseudo3D

[TatjanaWeiler]

inPoly function is not final yet

[boriskaus]

This generally looks fine to me. I've changed the name of the function to address issue #77
A few additional additions are required:

1. Please add help info to the function by using the """ ... """ before the function (see AddBox! for example).
2. Please add a test for this, as part of the test suite in test_setup_geometry.jl
3. Can you expand the tutorial to a real tutorial by adding some more info? You can use Literate.jl to create a markdown file from this and add one of more figures. Tutorials are under docs/src/man with images under docs/src/assets/img. Also add the new tutorial in the docs/make.jl file and it will appear in the sidebar of the tutorials. As an example for this, you can have a look at tutorials/Tutorial_Jura.jl

[boriskaus]

What’s the current status of this? Is it finished?