Shard Merge tool

[JMMackenzie]

This adds a tool for merging an index with multiple shards into an index with a single shard.

Help is required to test (write tests) and verify the tool.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.

Project coverage is 70.02%. Comparing base (1af285b) to head (a4574d4).

Files with missing lines	Patch %	Lines
src/main/java/io/anserini/index/MergeShards.java	0.00%	17 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2776      +/-   ##
============================================
- Coverage     70.12%   70.02%   -0.10%     
  Complexity     1316     1316              
============================================
  Files           186      187       +1     
  Lines         11922    11939      +17     
  Branches       1414     1415       +1     
============================================
  Hits           8360     8360              
- Misses         3035     3052      +17     
  Partials        527      527              

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[vz-gh]

@JMMackenzie I will take a look and test to get this merged soon. @b8zhong has added some tests and I will correct, update as necessary. Thanks for this again 👍
What indexes are you using with it? It might also help to have some reference.

[JMMackenzie]

I usually use Anserini to build CIFF indexes that I import to other tools like PISA, but I almost always forget to use -optimize when I index. So this tool fixes that.

You could try it on MSMARCO-v1 and MSMARCO-v2 (passages) if you wanted something large, not sure how long it would take for v2 though.

Thanks for your help!

[lintool]

Actually, I approved before noticing broken tests. Should probably fix before merging...

[JMMackenzie]

I'm not sure why these tests fail - I can't seem to generate any meaningful output. It seems the VM is crashing (I get "The forked VM terminated without properly saying goodbye" from surefire). Someone with more experience may be able to find a solution.