Introduce rounding paramter for cylinders

[hmeyer]

Cylinders are missing a rounding parameter (r???) - like boxes have one. maybe a general rounding parameter should be introduced, like "R" or "round"...

[colah]

Yeah, the only reason this isn't here is syntax...

There's also some cases, like rotate_extrude, where there's multiple notions of rounding we might have. I'm open to suggestions. In particular regarding consistency and backwards compatibility. (R or round are possibilities.) Maybe bring this up on the mailing list?