Returning paper number temporarily makes sum incorrect

[chrisklus]

In phetsims/number-play#23, I generalized the target nodes from ExplorePanel into CountingCreatorNode (to replace OnesCreatorNode in Number Play). While doing so, I moved the max sum of 9999 from the explore panel to instead be the max of the range of sumProperty in MakeATenExploreModel, which felt like a nice improvement (and it made for a consistent way to get the max sum value in CountingCreatorNode).

However, I discovered that when a paper number is returned to the ExplorePanel, its value is actually temporarily added to the current sum before it is subtracted from the sum. So for example, if i make the max sum 400, when I peel off 100 from the 400 in the explore area and try to return it, an assertion fires from the model sumProperty that it got a value of 500 (seen in the console):

I don't have time to investigate more now, but it seems reasonable that the sumProperty should never exceed its intended maximum value. I have a recollection of coming across this during the initial sprint for Number Play when trying to sync up the sumProperty of paper numbers and the currentNumberProperty from the Number Play model. Marking as blocks-sim-publication.