BEP: make `dir` optional for other than fmap modalities

[bids-standard]

BEP: make `dir` optional for other than fmap modalities

[kaczmarj]

@chrisfilo - i agree with @yarikoptic. dir should be valid for all epi, or it shouldn't be a tag at all.

it's confusing to have dir mean phase encoding direction for field map, and then acq potentially mean phase encoding direction for dwi, bold, etc. i am bids-ifying a study that uses AP and PA direction for dwi and bold, and it's inconsistent to specify the phase encoding direction with dir- for the field maps but acq- for dwi and bold, when they represent the same information.

[chrisgorgo]

I am a bit skeptical:

• it makes the filenames potentially longer
• creates two ways to describe the same thing (acq-PA and dir-PA)

I'm also not sure there would be such a big benefit from this change in terms of usability.

[yarikoptic]

Length stays the same since len(acq) == len(dir), unless there is also actually something else to say about acq.
acq is too generic/non-specific, dir meaning is consistent with current use in fmap, and potentially would be used for the same purpose.
To me it is about clarity - again acq can't be interpreted pretty much, while dir could

[chrisgorgo]

Please make a pull request - we can see if anyone feels strongly against this.