Cell results difference improvements

[RezaeiDoust]

Scenario 1: No difference ( normal use ) "Difference Case" --> None and "Base Time Step" should be come grayed out.

Scenario 2: Difference between different time steps in a single model. the same model name should be selected in "Difference Case". "Base Time Step" becomes active with " Select time step" being shown.

Scenario 3: Difference between two models: Item "same time step" (or something else) should also be able to be selected. i.e difference between two models and same time step