Consolidate fragments across MODIS versions

[gadomski]

As I work with 006 data, its really not different enough from 061 data to justify different fragment files. We should consolidate all fragments to just the product level.

• Supercedes Add keywords for 006 collections #59 because we won't need it anymore
• Requires Add modis:version attribute #83 so we can track what version each item is at the top-level