Added example in metatensor showing usage #26

Job	Run time
conda / conda (ubuntu-20.04)	10m 34s
docker / docker (fedora39)	22m 17s
codecheck / codecheck	44m 7s
conda / conda (macos-12)	18m 5s
docker / docker (rocky8)	23m 15s
linux / linux (-doc-mpi-)	37m 14s
conda / conda (macos-14)	6m 54s
linux / linux (-mpi-)	49m 42s
mac / macports (macos-12)	14m 56s
linux / linux (-coverage-mpi-)	1h 2m 0s
mac / macsimple (macos-12)	9m 0s
mac / macports (macos-14)	6m 40s
linux / linux (-debug-)	29m 30s
mac / macsimple (macos-14)	4m 46s
mac / macports (+allmodules, macos-12)	19m 43s
linux / linux (-debug-mpi-)	39m 32s
linux / linux (-intel-)	44m 54s
linux / linux (-pycv-mpi-)	17m 28s
	7h 40m 37s

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