revise sliders

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These should be the intervals for sliders:

Spring Constant sliders:

• [x] thumb interval = 10
• [x] tweaker interval = 1

Applied Force sliders:

• [x] thumb interval = 5
• [x] tweaker interval = 1
• [x] display min and max as "-100" and "100" (no decimal place)

Displacement slider:

• [x] thumb interval = 0.05
• [x] tweaker interval = 0.01

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The way that various related constants are specified in HookesLawConstants is not at all clear. Revise to use names like SPRING_CONSTANT_THUMB_INTERVAL and SPRING_CONSTANT_TWEAKER_INTERVAL. And have something like ROBOTIC_ARM_DRAG_INTERVAL for displacement controlled by the robotic arm.

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I adjusted the slider intervals. But HookesLawConstant.APPLIED_FORCE_DELTA needs attention. It looks like the related option (appliedForceDelta in Spring) was used at one time, but is no longer used (see https://github.com/phetsims/hookes-law/commit/115852b52e3d06988e3b2c719a2751aad986a6db). Can it be deleted? Or is it something that needs to be restored when I address #57 ?

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Merging this issue into #57, since this deals with more than just slider and tweaker intervals.