Fixed initial_items using tuple for NdMapping
NdMapping only allows deep indexing into items with __deep_indexable

Introduced __deep_indexable attribute for NdMapping and views objects.
NdMapping now only allows deep indexing into data elements with this
attribute.
Added items method to the NdIndexableMapping class
Casting initial_items for NdMapping objects to map_type before update
Simplified, fixed and cleaned up ndmapping