Get all loaded asset names

[McCallisterRomer]

• Adding method to NcAsset to get all currently loaded assets. This will be used during scene serialization.
  • Also adding a method for getting the AssetType from a service because it was a clean way to handle this and may be handy in the future.
• Adding missing default loading for skeletal animations (even though we aren't passing in a default yet).
• Adding a simple test for NcAsset. Not really much else to test until the other functions are migrated to the module.