Only check alternate node type if desired isn't present

[manzt]

This PR rearranges the control flow in open_array and open_group when mode is a, r, or r+, so that fewer store methods are called when the correct node already exists. The previous logic always required two calls to store.__containsitem__ if said array/group already existed. Now, open_array checks if the node is a group only if it is not an array (and vis versa for open_group). This may seem like a minor nit, but store.__containsitem__ can wrap arbitrary complexity and if it hides a network call, you can avoid an unnecessary network request.

I noticed this in the zarr.js implementation because we get a lot of HTTP 404s errors for .zgroup in the console when opening a valid remote array. gzuidhof/zarr.js#73

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• AppVeyor and Travis CI passes
• Test coverage is 100% (Coveralls passes)

[codecov]

Codecov Report

Merging #696 (6510e36) into master (acf6828) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #696   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files          31       31           
  Lines       10606    10606           
=======================================
  Hits        10600    10600           
  Misses          6        6           

Impacted Files	Coverage Δ
zarr/creation.py	100.00% <100.00%> (ø)
zarr/hierarchy.py	100.00% <100.00%> (ø)

[joshmoore]

👍

Merging and just going to suppress my inclination to try to refactor this into a single method call ;)