Set missing units

When validating or building a model, Openbox checks for any calculations without units. If it finds any, it will offer to work out, for each one, what the units should be. For example, if there is a calculation which adds two others with units of $, Openbox will set the units to $.

If it cannot work out what the units are, either because the formula is too complex or does not make sense, it will leave them empty.