parameterized hexagonal mesh

This model is a parameterized hexagonal mesh. I offer it primarily for training purposes with OpenSCAD.

This code was written intended as a negative in another object

skills demonstrated in this code:
- looping with `for`
- if
- rotate
- translate
- using cylinders with $fn=n$ as regular n-gons
- modules calling modules
- default argument values

amazingly, this code doesn't use any globals!

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suggested exercises:
- modify the code so that it lets you specify any number of columns, not just pairs of them
- modify the code so that the thickness of the hexes is a function of their position in the grid