CMIP6 files in the wild use fractal heaps with huge instances and internal b-tree indexes

[bnlawrence]

The current code does not implement a particular kind of fractal heap; we have (in the Fractal_Heap class within misc_low_level.py:

if header["btree_address_huge_objects"] == UNDEFINED_ADDRESS:
            header["btree_address_huge_objects"] = None
        else:
            raise NotImplementedError

But we see data in the wild (in particular in CMIP6) which uses this packing, so we need to support it.