Make either 2D or 3D positions a requirement

[rob-luke]

Closes #65

This changes the specification to require either 2D or 3D positions, not just 2D.

[dsleiter]

Good to hear this is being implemented!

To ensure internal consistency within the document, the "SNIRF data format summary" section should probably be updated as well. There isn't an existing notation for "one of" - one option that comes to mind is: *ⁿ (with *¹ for source positions and *² for detector positions)

• *ⁿ in the last column indicates that at least one of the subfields in the subgroup identified by n required

But maybe there is another simpler notation.

[fangq]

use detectorPos2D to quote SNIRF field names.

also need to add a notation, say "**" or "+" in the summary table for the relevant fields

https://github.com/fNIRS/snirf/blob/master/snirf_specification.md#snirf-data-format-summary

for some reason, I feel that this solution is a bit cumbersome for future validator's/parsers, but don't have a better suggestion at this point.

[rob-luke]

Thanks for the review. @fangq does this look better?

[fangq]

looks good, thanks

[rob-luke]

Thanks for the feedback @dsleiter and @fangq