Stellarator non-symmetric configurations in vmec_splines

[daringli]

Adds support for stellarator non-symmetric configurations in vmec_splines. A notable example of such a configuration is ITER, where the lower-null divertor breaks stellarator symmetry.
For stellarator symmetric configuration, the additional outputs are all zero.

[daringli]

Your commit looks like a strict improvement. Thank you for making these changes.
The X, Y, Z, etc. outputs from vmec_compute_geometry had snuck in from a previous edit that had nothing to do with this. Thanks for removing them.