polygon pot

Simple openscad of a pot with a spiral option.

stl files:
* pot_7.stl - seven sided, 50mm bottom, 90mm top, 45mm height
* pot_12.stl - twelve sided, 50mm bottom, 90mm top, 45mm height

I notice that thingiverse customiser appears to have a problem with 7,11,14,17,21 and 22 sided pots. This may just be a rendering problem. Openscad 2021.01 had no problems.

Configurable:
* number of sides
* diameter
* twist between layers
* increase in pot diameter with height
* wall thickness
* bottom thickness
* number of segments in height
* text option on the underside
* foot instead of flat bottom
* foot and text don't work well together unless you use a thick bottom