merge: Support sequences with cross-checking

[victorlin]

Description of proposed changes

Initial prototype for sequence support in augur merge where metadata and sequence merge happens with additional cross-checking.

Related issue(s)

Closes #1579

Checklist

• Address FIXMEs
• Add seqkit as an external dependency (c099e16)
• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

Attention: Patch coverage is 59.37500% with 13 lines in your changes missing coverage. Please review.

Project coverage is 70.99%. Comparing base (da1c89d) to head (d143872).

Files	Patch %	Lines
augur/merge.py	59.37%	7 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1601      +/-   ##
==========================================
- Coverage   71.02%   70.99%   -0.03%     
==========================================
  Files          79       79              
  Lines        8256     8286      +30     
  Branches     2003     2013      +10     
==========================================
+ Hits         5864     5883      +19     
- Misses       2101     2107       +6     
- Partials      291      296       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.