ENH: improvements for using `bids_dataset` extractor in workflows

[jsheunis]

This PR:

• Improves results handling of get_required_content: yields impossible results instead of raising exceptions; yields results for more test cases than before (addresses https://github.com/datalad/datalad-neuroimaging/issues/122)
• Removes requirement of participants.tsv file (addresses https://github.com/datalad/datalad-neuroimaging/issues/121)
• Removes the option to search for the BIDS dataset root location further down in the filetree of the dataset passed to the meta-extract command, since this caused problems when using the extractor in a workflow on super-sub-dataset hierarchy. See https://github.com/datalad/datalad-neuroimaging/issues/123, for which the outcome may lead to an additional PR.

[codecov[bot]]

Codecov Report

Patch coverage: 23.80% and project coverage change: -0.46 :warning:

Comparison is base (c1721c9) 89.61% compared to head (83a6c29) 89.16%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #124 +/- ## ========================================== - Coverage 89.61% 89.16% -0.46% ========================================== Files 24 24 Lines 1175 1181 +6 ========================================== Hits 1053 1053 - Misses 122 128 +6 ``` | [Impacted Files](https://codecov.io/gh/datalad/datalad-neuroimaging/pull/124?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | Coverage Δ | | |---|---|---| | [datalad\_neuroimaging/extractors/bids\_dataset.py](https://codecov.io/gh/datalad/datalad-neuroimaging/pull/124?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9uZXVyb2ltYWdpbmcvZXh0cmFjdG9ycy9iaWRzX2RhdGFzZXQucHk=) | `81.88% <23.80%> (-3.73%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad)

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