Automate J2 cusp value in case of spin-orbit

**Is your feature request related to a problem? Please describe.**
Current code set J2 spin-like cusp value to `-0.25` by default but this is not a good value for SO.
**Describe the solution you'd like**
Some change can be made in the J2 builder to detect if SO is in use and set the value to `-0.5` by default.
In addition, there is a missing check probably to stop more than one species in the electron particle set when SO is in use.