[bug] fix #103--unused scenarios not omitted by preprocessor

If some scenarios are omitted by the dataset_splits (either explicitly by adding an 'unused' split or implicitly when the splits don't sum to 1.0) make sure to actually omit those scenarios from the final preprocessed scenario params table.

This was the intended behavior for this case when I implemented !96 (merged) but it was not properly tested. Now there is a test ensuring this behavior.