Re-factor getBlocks()

In the collection object:
- getBlocks() re-factored to get a list of blockIDs using a separate method getBlockIDs().
- added doc comments to the methods concerned

This provides an external interface to get a list of bIDs in an area without needing to get the actual block objects.
Thus enabling enquiries that only need bIDs to be considerably more efficient

The change only involved splitting existing code into another method.