Update scattering lengths for lithium and rare earths.

[pkienzle]

New bound coherent scattering length measurements are available:

• lithium: Gehlhaar (2025) https://doi.org/10.1088/1361-648X/add3a6
• various rare earths: Gehlhaar (2026) https://doi.org/10.1088/1361-648X/ae1ec0

I have updated the values in python-periodictable here.

[pkienzle]

Also, 13C here: https://doi.org/10.1088/0953-8984/20/04/045221