Estimate development effort for next phet-io milestone

[samreid]

@kathy-phet requested an estimate of developer hours to prepare the next phet-io milestone. I said:

If I can wait until @arouinfar reports on the tree, I can give a more accurate development estimate.

But I also opened this issue to put it on my todo list.

[arouinfar]

@samreid I've completed an in-depth review of the Net Force screen. It's taken about 2.5 hours, and I estimate it will take a similar amount of time to complete the remaining 3 screens (since they are all similar). I plan to continue that tomorrow. My notes can be found in the PhET-iO doc. I'd be happy to meet and go over it together, or feel free to read through it on your own. Let me know what you prefer.

I will eventually convert the doc over to issues for #294, but that takes quite a bit of time and doesn't seem necessary for the purposes of your estimate.

[samreid]

I provided a development effort estimate in the slack thread, closing.