Add a warning if people use data formats that are "allowed but not recommended"

[choldgraf]

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In the iEEG spec, we agreed that some data formats should be allowed, though not recommended. This is meant to give us some more flexibility on deprecations and "official" support as the landscape around data formats changes (it is much less-stable than the fMRI landscape).

In bids-standard/legacy-validator#591 we discussed having the validator raise a warning whenever a user includes data in one of these "allowed" but not recommended formats. This is an issue to keep track of whether implementing this feature is a good idea.