Dont walk arrays

[will-moore]

NB: this is on top of #9.

Fixes #10.

This avoids traversing all the chunk directories under .zarray directory, which we know will only contain chunks.

[will-moore]

Testing... installed on idr-testing.

(venv3) [root@test120-omeroreadwrite wmoore]# pip install git+https://github.com/will-moore/omero-mkngff.git@dont_walk_arrays

And on idr0125-pilot:

(venv3) [root@pilot-idr0125-omeroreadwrite wmoore]# pip install git+https://github.com/will-moore/omero-mkngff.git@dont_walk_arrays
Collecting git+https://github.com/will-moore/omero-mkngff.git@dont_walk_arrays
  Cloning https://github.com/will-moore/omero-mkngff.git (to revision dont_walk_arrays) to /tmp/pip-req-build-a9c45z58
  Running command git clone -q https://github.com/will-moore/omero-mkngff.git /tmp/pip-req-build-a9c45z58
  Running command git checkout -b dont_walk_arrays --track origin/dont_walk_arrays
  Switched to a new branch 'dont_walk_arrays'
  Branch dont_walk_arrays set up to track remote branch dont_walk_arrays from origin.
  Resolved https://github.com/will-moore/omero-mkngff.git to commit 7db8b499708112ea3f250c1bbf82e920c269d8d3

[will-moore]

Updated again on idr-testing and idr0125-pilot to include last commit.

[will-moore]

Tested today with idr0013. IDR/idr-metadata#644 (comment)
3 minutes per fileset

[will-moore]

For info about that last commit, see IDR/idr-metadata#644 (comment)