Describe the new feature

A common pattern we have developed in our plant models is to calculate how much Tt remains between stages as shown in the screen shot below.

This approach is a bit cumbersome and error prone when copying functions to duplicates.

Would be better to have a dedicated function to calculate the Tt remaining. Would only need to have inputs of the stage names (with drop downs so it is not possible to get them wrong) and the rest can be done in code so there is no possibility to make config errors in the json model set up.