Poor man's openscad screw library (revised)

This is a remix of the great [library](https://www.thingiverse.com/thing:8796) by [aubenc](https://www.thingiverse.com/aubenc/about).

Features:
* Add screw_lid() and hollow_screw_thread() modules that can be used for making a lid and threaded tube or even a threaded jar. (see example_6 in code).
* Adds documentation of input variables into the source code.
* Adds examples into source code, with switches at top to turn examples on or off
* Updates OpenSCAD syntax to avoiding compiler warnings. (I note that [this](https://www.thingiverse.com/thing:1344958) remix also does this.