Select differentials by drag-and-release

At present, the only way to select a different is to change to the appropriate page and click the source term cell several times under the source term cycles through to what you want.

If functionality supporting #84 were implemented, then it would also be easy to add another path for selecting a differential: click and drag from a source term to a target term. Then, EXTLocation's +calcluateDifflPage could be used to turn to the appropriate page and select (or construct) the indicated differential.