Parametric Rocket Fin Alignment Jig

This is a remix of justblair's awesome fin alignment jig. The original had a few minor problems which I've fixed (most of these affected larger jigs in particular):
- fin width is now calculated correctly (it was previously half a tube diameter off)
- the fin tab ends now respect the wall thickness setting
- the brim width is now configurable
- the cylinder smoothness variables have been modified from the $fn method to the $fs/$fa method
- the center