I think that we should change the name of this variable to be more indicative of what it holds.  A "hash map" or a "hash table" is a generic term for a specific, different data structure.  (`std::map<T, U>` is a binary tree, dictionaries are hash tables.)  I suggest `nuc_data_hashes` or `dataset_hashes` or `data_checksums` or maybe just `checksums` since we are in the `pyne::data` namespace.  I leave it up to you though @crbates.