Issue with Tb

There is an issue with Tb, where entries in the magnetic dipole matrix become NaN. To the best of my knowledge, this comes from Tb's slj values.

Code to reproduce:
```
from NanoParticleTools.inputs.spectral_kinetics import SpectralKinetics
from NanoParticleTools.species_data import Dopant
sk = SpectralKinetics([Dopant('Tb', 0.1, 3)])
sk.magnetic_dipole_rate_matrix
```

Output:
```
array([[0. , 0. , 0. ],
[ nan, 0. , 0. ],
[0. , 1.20341175, 0. ]])
```

Run the provided reproduction using SpectralKinetics and Dopant, then inspect the magnetic_dipole_rate_matrix path and Tb's slj values in the species-data implementation. Trace where the NaN enters the matrix and verify that the reproduction produces finite rate values after the issue is fixed.

Written by the indexing model from the issue text.