data.table names handling for unnamed matrix

[jangorecki]

Closes #4124

[jangorecki]

@MichaelChirico this PR addresses issue raised by you. There are few tests that needs to be reviewed.

[MichaelChirico]

Thanks for the ping. The PR looks good. I can't see which tests failed, would you mind updating the PR so at least they pop out on CI?

[github-actions]

• HEAD=dt-matrix-names stopped early for fread disk overhead improved in #6925
• HEAD=dt-matrix-names slower P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit c502cee

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 58 seconds
Installing different package versions	39 seconds
Running and plotting the test cases	2 minutes and 39 seconds

[MichaelChirico]

I stripped out the part that fixes #3193 -- in retrospect, I would close that issue unfixed, you're right that it derives directly from data.table's default of check.names, which I am a bit loath to change. At a minimum, it needn't be addressed here.

Tests added since this PR was first opened expose another breaking change:

https://github.com/Rdatatable/data.table/blob/94f4081b9f8be13192fd8a43d0f721e246d5c347/inst/tests/tests.Rraw#L17437

data.table(matrix(1L, dimnames = list('a', 'b')))

## master
#        b
#    <int>
# 1:     1

## this PR
#       V1
#    <int>
# 1:     1

I think we should continue to respect column names of a 1-column matrix input.

[MichaelChirico]

Good to go from my side. Jan, PTAL and merge when you're ready given my edits.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (94f4081) to head (c502cee).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4363   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          79       79           
  Lines       14687    14689    +2     
=======================================
+ Hits        14496    14498    +2     
  Misses        191      191           

☔ 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.