[ADD] Motion (BEP029) example data for system validation containing two motion datasets

[JuliusWelzel]

@sjeung I did put the motion example for the system validation dataset on a different branch to make merging easier

[sappelhoff]

:point_up: for the record, the additions for motion should all be done like so:

1. checking out the bep029_motion branch (in this repo, see: https://github.com/bids-standard/bids-examples/tree/bep029_motion)
2. checking out a new "feature branch"
3. making changes on that feature branch
4. opening a PR against bep029_motion

This PR apparently has been done like:

1. checking out a feature branch from master
2. making changes
3. opening a PR against master

(I know that because when I tried changing the PR "base" as shown in the screenshot, the "files changed" number went up dramatically)

If you know how to easily conform to the above mentioned way (using the bep029_motion branch), I'd appreciate that. But don't worry if you don't know how or if it's too much of a hassle.

I think we need to document the workflow more clearly

cc @JuliusWelzel @sjeung

for the other two PRs, this has worked well, BTW.

The advantage of the proposed method is, that we can also merge your example PRs a bit sooner and "collect" them in the bids-standard/bep029_motion branch ... and then we we get close to integrating motion into BIDS, we simply do a single PR from bep029_motion into master

[sappelhoff]

@JuliusWelzel can this one be close now that you have #347 open?

[JuliusWelzel]

Closed because new example dataset PR moved(https://github.com/bids-standard/bids-examples/pull/347)