Fix & improve .window_size validation

[brookslogan]

Checklist

Please:

• Make sure this PR is against "dev", not "main" (unless this is a release
  PR).
• [-] Request a review from one of the current main reviewers:
  brookslogan, nmdefries.
• Makes sure to bump the version number in DESCRIPTION. Always increment
  the patch version number (the third number), unless you are making a
  release PR from dev to main, in which case increment the minor version
  number (the second number).
• Describe changes made in NEWS.md, making sure breaking changes
  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  1.7.2, then write your changes under the 1.8 heading).
  • (going to merge others first & figure out right number)
• See DEVELOPMENT.md for more information on the development
  process.

Change explanations for reviewer

• Fixes where time_type = "day" would allow any type of difftime.
• Complete docs and match code&messages wrt .window_size validation
  • Mention what's acceptable for yearmonth time_type
  • Mention Inf in validation errors as acceptable iff it's actually acceptable.
  • Reject any other strange "int" classes that pass test_int. (It rejects Date
    and POSIXt, but perhaps there are others.)
  • Refactor to use helper function test_sensible_int instead of test_int (as
    latter accepts difftimes and makes logic look confusing).-

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves #{issue number}

[dshemetov]

Ah, I'm seeing a bunch of minor improvements to the slide args I didn't move over to epi_slide_opt, I can add a commit in a little. Thanks for noticing!

[brookslogan]

Will you be removing .window_size = 1 default as part of the epi_slide_opt sync? I was originally setting out to do that and got distracted, oops. [Wait, no, I removed the .window_size = 1 default but didn't test that it's actually complaining if given NULL... I didn't mean to commit that yet or in this PR...]

• todo: actually check

[dshemetov]

Yea I didn't port the validation from epi_slide over epi_slide_opt, but I'll do that now.

[dshemetov]

Ok looks better now. Should be good to go.