WIP: Improvements from user suggestions

I'm not a huge fan of doing it this way because `ScalarModel` needs to now keep track of the x_index.  The first thing I tried was `index << scalar_collection.col_names.index(scalar_collection.x)` which did trigger when `scalar_collection.x` was changed. Unfortunately it had some unexpected behavior since it triggered whenever `scalar_collection.col_names` got changed too, which is primarily a problem [because of this line] (https://github.com/Nikea/replay/blob/master/replay/model/scalar_model.py#L219).  When I reset col_names to an empty list, `line 54` here throws an exception.  Perhaps this is indicative that I'm abusing Atom/Enaml?