HDF5 refactor

[d-chambers]

Description

This PR fixes #278 by adding support for the Brady Hotsprings flavor of CF1.7 and implementing the ideas in #290, although the interface is simpler. A consequence is now either pytables or h5py can be used for file io.

Also fixed a small bug with how attrs extracts dimensions from coordinates, and simplified the internals of resource manager.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings or appropriate doc page.
• included a test. See testing guidelines.
• your name has been added to the contributors page (docs/contributors.md).
• added the "ready_for_review" tag once the PR is ready to be reviewed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ec2c248) 99.30% compared to head (1bb6090) 99.47%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
+ Coverage   99.30%   99.47%   +0.16%     
==========================================
  Files          86       88       +2     
  Lines        6796     6850      +54     
==========================================
+ Hits         6749     6814      +65     
+ Misses         47       36      -11     

Flag	Coverage Δ
unittests	99.47% <100.00%> (+0.16%)	⬆️

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