utils: Restore read_metadata and supporting file

[victorlin]

Description of proposed changes

This reverts commit 040822c.

As discussed in #970, there are still some references to augur.utils.read_metadata in core Nextstrain workflows.

See #970 (comment) for plans to add this change back.

Related issue(s)

• Document how to upgrade read_metadata #970
• utils: Remove read_metadata() in favor of io.read_metadata() #934

Testing

See checks.

[codecov]

Codecov Report

Merging #972 (7a201a6) into master (0d3f4c4) will increase coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #972      +/-   ##
==========================================
+ Coverage   58.99%   59.31%   +0.32%     
==========================================
  Files          49       50       +1     
  Lines        6201     6250      +49     
  Branches     1575     1582       +7     
==========================================
+ Hits         3658     3707      +49     
  Misses       2283     2283              
  Partials      260      260              

Impacted Files	Coverage Δ
augur/util_support/metadata_file.py	100.00% <100.00%> (ø)
augur/utils.py	64.83% <100.00%> (+0.45%)	⬆️

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