WavelengthSlider needs additional options

[pixelzoom]

Noted while working on models-of-the-hydrogen-atom.

If you decrease options.trackHeight, then the value may collide with the buttons, e.g.:

So it would be nice to be able to control the size of the tweaker buttons, and the vertical spacing between the bottom of the value and the top of the track.

[pixelzoom]

Added options.ySpacing and options.maxTweakersHeight, working nicely in MOTHA:

@samreid Would you mind taking a peek?

[samreid]

All 3 commits look good, my only question is whether we would like a more specific term in place of ySpacing, perhaps something like verticalValueDisplayMargin? It seems like ySpacing runs the risk of colliding with some future term.

[pixelzoom]

Changed ySpacing to valueYSpacing.

[samreid]

Sounds good, closing.