[ENH] Extend date time information to include optional UTC syntax, warn about FIF requirements

[sappelhoff]

In https://github.com/bids-standard/bids-validator/pull/1019 I wanted to add the validation of the newly introduced "fractional seconds" that we allow for AcquisitionTime in scans.tsv files.

While discussing that code, we stumbled over potential incongruencies between RFC3339 and the way that standard is implemented in BIDS.

• particularly with regards to the timezone (see https://github.com/bids-standard/bids-validator/pull/1019)

In this PR we may clarify/fix some aspects ... or in the least we'll add a link and fix a typo (see
de4ab9f )

EDIT: we are now adding optional Z syntax to indicate UTC time in accordance with RFC3339

• adjust validator for Z syntax

[sappelhoff]

I now also link to the Units section from the modality specific MEG, EEG, and iEEG sections to

1. avoid duplicating explanations
2. drop the unspecific reference to ISO8601 (we are using a specific subset of this standard)

while doing that, I found the following sentence:

It does not need to be fully detailed, depending on local REB/IRB ethics board policy.

repeated across the files.

For now I dropped it from the three files and added it to the Units section. However, I think we can drop this sentence completely, because it is more or less covered in the point on anonymization that we have.

[tsalo]

Just some minor edits, but it looks good to me.

[effigies]

Two very minor suggestions.

[tsalo]

LGTM.

[franklin-feingold]

lgtm thanks @sappelhoff