BUG: fix loading large npz files (fixes #2922)
Chunk reads in format.read_array.

Maximum data size limitations in the crc32 module cause errors when
reading more than 2 ** 32 bytes from gzip streams. Work around this
issue when reading large arrays from npz files by chunking reads to
256mb.

This appears to resolve bug #2922.
adjust the optimal IO buffer size for npz files
Merge pull request #2942 from btel/npz_optimize

BUG: Fix loading npz files >2GB on 64bit systems