New magnetic field API #1807
Description
This issue documents the new API for magnetic field computations that I discussed at the last developer meeting. The other developers agreed to it. The current API makes analysis difficult, especially for quantities that are derived from the magnetic field, and slows feature development.
The new API in #1805 will register computable quantities in the same manner that the equilibrium class does. The old magnetic field API for existing classes will be updated to the new ones in a later PR.