Adds controls for fniycbo, the core drift currents

[holm10]

• Introduces the variable bbb.isfniycbozero for controlling fniycbo
  • If bbb.isfniycbozero=1, the drift currents over the core boundary are averaged to zero
    • Redistributes the current profile in the core
    • Can be continuously varied to transition smoothly
  • If bbb.isfniycbozero<0, sets fniycbo=0 on core, e.g. no drifts currents are considered