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.
Merge pull request #1 from asford/npz_optimize

Chunk reads in format.read_array.