Allow each view to include a fragement ID column

[jkerr5]

Currently, only the "primary" view (the first one in the data sources array) can include the fragment ID column and it is hardcoded to DocId.

Change this so each view also selects its own fragment ID column and places it into a named column that is specific to that view. The name of the column can be specified in the data source section of the layer descriptor via the fragmentIdColumn property.

If that property is not present, a naming convention will be used and the fragment ID column name will be <view Name>_DocId.

Per issue #61, the geometry.source.joinDocColumn could refer to the fragment ID column from one of the views.

[jkerr5]

Need to look at this closer to see if it is still an issue.