Clarify a few documentation items.
LatentHeat: rm “Reset Experiment” button & disable ‘Start adding heat'

Remove one of the Reset buttons from the Latent Heat interactives.

Disable 'Start adding heat' button in Latent Heat interactive after the first time it is clicked.

[#61563022][#61563190]

https://www.pivotaltracker.com/story/show/61563022
https://www.pivotaltracker.com/story/show/61563190