Adds `metadata/annotations/anatomy` to the 16 human files in `us4us/data/`: `s<N>_l.hdf5` -> `left forearm`, `s<N>_r.hdf5` -> `right forearm`.
Source: the README ("left and right human forearms of 8 healthy volunteers"); the side is taken from the filename suffix. `reference_water.hdf5` and the `yezitronix-b-rg-1.2.hdf5` phantom are unchanged.
Files changed: 16. Only `metadata/annotations/anatomy` was written, `raw_data` and all other metadata are untouched.
Diff:
```
us4us/data/s1_l.hdf5: differs (main)
+ metadata/annotations: (group)
+ metadata/annotations/anatomy: 'left forearm'
us4us/data/s1_r.hdf5: differs (main)
+ metadata/annotations: (group)
+ metadata/annotations/anatomy: 'right forearm'
us4us/data/s2_l.hdf5: differs (main)
+ metadata/annotations: (group)
+ metadata/annotations/anatomy: 'left forearm'
us4us/data/s2_r.hdf5: differs (main)
+ metadata/annotations: (group)
+ metadata/annotations/anatomy: 'right forearm'
us4us/data/s3_l.hdf5: differs (main)
+ …
