Extend the Asset model

[etj]

Currently Asset are only modeled to contain data and be linked to one (or more) resources.
It mainly resembles the previous model where a resource is linked to its own set of files

As we're moving toward a more complex handling of Assets, we need some more info about how Assets are related to Resources.

• Mandatory Assets:
  • a mandatory asset is an asset that can not be unlinked/delted from a resource. For instance, the Geotiff file in a Raster dataset
• Ordering
  • we may want Assets to be ordered in a certain way within a Resource view
  • note that an Asset may be the 1st in a resource but the 10th in another
• Private Assets
  • a private assets may contain service data, useful for the resource owner but not published (for instance a log for some internal processing)