Use VerticalCheckboxGroup instead of custom ISLCCheckboxItem

[zepumph]

I'm unsure why there is a custom is solution here, even after looking at #51.

It seems like we should be using VerticalCheckboxGroup, I'll take a look further. From phetsims/gravity-force-lab-basics#134

[jessegreenberg]

I think the motivation for removing VerticalCheckboxGroup in #51 was because VerticalCheckboxGroup does not currently support disabling a single checkbox in the group.

[zepumph]

@jessegreenberg in the above commits I added support to VerticalCheckboxGroup to support individual options passed to a single checkbox. From that it was easy enough to pass a custom enabledProperty to the scientific notation checkbox. Please review. Note that this changed the layout of the checkbox panel by a few pixels (not greatly). What do you think about this change?

[jessegreenberg]

Awesome, thank you! What do you think about changing the order of options for the extend call to

_.extend( {}, options.checkboxOptions, item.options, {

so that the group options can be overridden by individual options?

[zepumph]

Great idea! I also needed to do some cleanup in terms of how we were passing enabledProperty through (mainly for phet-io related problems). @chrisklus was a great help in simplifying. His idea was to just use the showForceValuesProperty as the enabledProperty, so there was no proxy/intermediary needed. @jessegreenberg anything else for this issue?

[terracoda]

@zepumph and @jessegreenberg, the disabling option has been working visually, but I don't think it has ever worked in the PDOM. In the screenshot you can see that the disabled check box is focusable and checked.

This checkbox issue is being tracked in phetsims/gravity-force-lab#110.

[zepumph]

Sounds good. @terracoda see phetsims/sun#517 for the general solution. Anything else for this issue?

[jessegreenberg]

Changes look great, and since the issue @terracoda mentioned is going to be tracked elsewhere I think this can be closed.