Adds sample selection to equivariant power spectrum

[bananenpampe]

Adds sample selection to the equivariant power spectrum convenience class.

[Luthaf]

Please don't import from metatensor.operations. This could be written

from metatensor import equal, join

Or even better

Suggested change

	from metatensor.operations import equal, join
	import metatensor as mts

and down the line mts.equal, mts.join

(like you would not do from numpy import log but instead use np.log where required)

[Luthaf]

Thanks, looks very good overall! I just have one style comment about importing metatensor operations.

[jwa7]

Thanks @bananenpampe !

[bananenpampe]

test makes the naming consitent with the other tests