ISLCModel.forceProperty has a todo

[jessegreenberg]

From #30, just marking this as an issue. units option for ForceProperty has this TODO:

units: 'Newtons' // TODO: this appears unused in instance-proxies

@mbarlow12 and/or @samreid does this need to be addressed?

[samreid]

This issue is caused by https://github.com/phetsims/phet-io/issues/1292 -- inability to specify units or other NumberProperty metadata for DerivedProperties. This doesn't need to be solved before 1.0, and I don't know how we'll proceed with https://github.com/phetsims/phet-io/issues/1292, but when I come up with a plan, I'll use this as a test case.

[jessegreenberg]

Thanks @samreid, in that case marking as "deferred" for the 1.0 release.

[jessegreenberg]

Linking this issue to the TODO.

[samreid]

Addressed above. Now the force appears in the PhET-iO Studio wrapper as a numeric read-only property with the appropriate units:

Also, this went through a medium-sized merge, @jessegreenberg or @mbarlow12 can you check that nothing is disturbed?

[jessegreenberg]

Thanks @samreid. Removing deferred label. @mbarlow12 could you review the change?

[mbarlow12]

Reviewed. All sims seem to be running fine as well. Closing.