Automate J2 cusp value in case of spin-orbit

[ye-luo]

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.

[camelto2]

For reference, if you use nexus it automatically sets the appropriate cusps for you. But I agree, this should be done inside qmcpack for those who don't use nexus.

[prckent]

Thanks. The internal QMCPACK defaults should be physically correct, so this is worth improving.