The simulator has advanced considerably, and it is seeing lots of active
development, so make it a required part of KiCad. Additionally, the
build without the simulator has actually been broken for a while, so no
one clearly is building without ngspice right now.
Follow-up after the KIBIS and KIBIS GUI merge requests.
- Move KIBIS from Pcbnew to Eeschema space,
- Make KIBIS obtain the Ku/Kd coefficients via the `SPICE_SIMULATOR` class instead of calling the `ngspice` executable via `system()`,
- Allow to toggle between differential and single-ended model in the GUI,
- Various GUI fixes and improvements.