allow passing zlib=True to write_nc to enable compression

I was surprised to see that write_nc created nc4 files of the exact same size of in-memory arrays. Turns out zlib compression is not enabled. With these changes one can call dataset.write_nc(..., zlib=True) to enable compression.

With these change my output files have been reduced from 9.4M to ~120K, which made me breathe easier (I need to create ~50,000 of those).

It's my first pull request so I might have done some things wrong :-)