RFE: add cartoon-style helix tube radius option

[Elaine Meng]

I recommend adding an option to “cartoon style” to allow specifying a constant radius for tubes (per-model). Sometimes the varying radii don’t look that good, and sometimes some look obviously too fat (1gmk chain C) or too thin (3e86 short C-term helices). I anticipate users will want to be able to control tube radius, and this would afford control as well as allow making them all the same.

Maybe the option could be:

radius [ auto | value ]

… or tubeRadius instead of radius if you prefer, would require more typing though
… or default instead of auto if you prefer, meaning the current method (the average distance from CA to tube midline, right?)

It would be a per-model attribute that only affected the helix tube mode.

[Elaine Meng]

Also need a way to make the radius persist over different frames of a trajectory, if possible (see #588 for example data).

[Conrad Huang]

Implemented in eff7774.

[Elaine Meng]

Another thought is should "auto" be the maximum distance rather than the average, to avoid floaters?