open...() functions should accept pathlib.Path

Minimal, reproducible code sample, a copy-pastable example if possible

from pathlib import Path
store = Path('/path/to/store')
zarr.open_group(store)

…gives:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-8-4a4272aec66c> in <module>()
----> 1 codon_position = zarr.open_group(output_dir / 'codon_position.zarr.zip')
      2 codon_position.tree()

~/github/wtsi-team112/vector-ops/deps/conda/envs/vector-ops/lib/python3.6/site-packages/zarr/hierarchy.py in open_group(store, mode, cache_attrs, synchronizer, path)
   1117 
   1118     elif mode == 'a':
-> 1119         if contains_array(store, path=path):
   1120             err_contains_array(path)
   1121         if not contains_group(store, path=path):

~/github/wtsi-team112/vector-ops/deps/conda/envs/vector-ops/lib/python3.6/site-packages/zarr/storage.py in contains_array(store, path)
     70     prefix = _path_to_prefix(path)
     71     key = prefix + array_meta_key
---> 72     return key in store
     73 
     74 

TypeError: argument of type 'PosixPath' is not iterable

Problem description

Functions that accept a string path for the store argument should also accept Path object.

Version and installation information

Please provide the following:

• Value of zarr.__version__: 2.2.0
• Version of Python interpreter: 3.6

[clbarnes]

Basic support was released in 2.9.0: https://zarr.readthedocs.io/en/stable/release.html#enhancements

I think this can be closed unless there’s desire to base more of the internals on Paths.

[LunarLanding]

Most software I’ve been using accepts path objects. Would a minimal pull request be accepted?