add sv2a pet atlas from NRU to examples

[bendhouseart]

Adds PET atlas to examples, will fail validator for version v1.11.1, fix coming in minor release v1.11.2.

[effigies]

You're going to add hemi for volumetric data? Or are you using the FS-NIfTI hack for surfaces?

Either way, just use GIFTI.

[CPernet]

I am quite certain the extension should be .mimap and not pet since the values are not PET values anymore (discussed many times in many different threads @mnoergaard

[effigies]

If we want to use mimap, we need to finish off bids-standard/bids-specification#2262.

[CPernet]

I agree - we cannot push a PET atlas with the wrong extension

[bendhouseart]

I agree - we cannot push a PET atlas with the wrong extension

We need to update the spec then as any suffix is allowed. I'd rather change the schema than try to meddle with the spec directly, but maybe that's just me. This PR works with these examples and complies with the spec, so it is possible to have both.