Add hiding Epsilon and/or Sigma controls to client guide

[KatieWoe]

For phetsims/qa#531.
Related to client request:

Hide the interaction strength slider

and these properties were given to do this:

statesOfMatter.phaseChangesScreen.view.moleculesControlPanel.interactionStrengthSlider.visibleProperty
statesOfMatter.interactionScreen.view.atomsControlPanel.interactionStrengthSlider.visibleProperty

In both these cases, the Epsilon control in the interactive graph allows changes without those sliders. This should be updated in the guide. I don't believe the Sigma control is relevant in the guide, but just in case.

[arouinfar]

Thanks @KatieWoe. This was a good find. I added the epsilon controls to the SOM guide (irrelevant to SOMB). Since the client didn't have any sigma-related requests, I didn't add the sigma graph controls to the guide.

Back to you @jbphet.

[jbphet]

changes cherry-picked to the 1.2 branch

[KatieWoe]

Looks good in rc.3