[Feature request] Hollow print option

[ben-oxley]

I'd like to create an option to export shelled STLs. Whilst this can be done as a postprocessing step, for detailed geometries, this can take quite a lot of time when it should be easy to do at export.

The option would be a boolean enable along with a thickness in mm to be used. this would still create walls around the side down to the bottom depth specified, but would be hollow underneath. This will enable easier printing for online services.

Note: I'm keen to contribute this feature. I have made a fork and intend to start working on it, but didn't want to blindside with a PR down the line.

[ben-oxley]

Potential solution implementation here: https://github.com/ben-oxley/DEMto3D-QGIS-Plugin/blob/shell-option/model_builder/STL_Builder.py#L149

Preview of intended effect: