Parametric Olympus reversible lens hood sized for 75-300

This is a remix of http://www.thingiverse.com/thing:240706 with a few enhancements:
- It separates the bayonet flange from the inner end of the hood, so the diameter can be enlarged for reversibility.
- There's a test mode (in the .scad version) that prints just the bayonet, for quick size testing.
- It's a bit shorter (but this is easily controlled by parameters).

This is currently sized for a good fit on the Olympus m4/3 75-300mm, but is probably adaptable to other Olympus variations.