Element.Geometry returns object - one element or an IEnumerable

### Issue

The Element.Geometry previously always returned an array.  Certain Elements return more than one piece of geometry, so this was required.

However, this meant users would have to extract the first item when the Element returns a list.  This is considered confusing, so this fix changed `Element.Geometry` to return a single Geometry when the list is length 1.  

Here's an example: 

![selectmodelelement](https://cloud.githubusercontent.com/assets/916345/4481421/a1124d30-499f-11e4-9c32-3dc0070e1146.PNG)

### Caveats

This means that all old workflows which extracted the first item will fail.  This also means that workflows won't be able to predict the type of output.  This also might be confusing.  

I should say that I'm not totally comfortable with this change, but here it is for your consideration.

### Reviewers 

@lukechurch @ikeough