Implement MultiCoil

[bclyons12]

This creates a new MultiCoil that contains a collection of coils assumed to be connected in series. It also creates a new type AbstractSingleType which is a subtype of AbstractType.

[bclyons12]

@orso82 Technically this is a breaking change, since AbstractCoil is now a supertype of the new AbstractSingleCoil. AbstractSingleCoil now acts like AbstractCoil used to (with typing and such). If someone had something like AbstractCoil{T1,T2,T3,T4} in their code, then this would break. That said, nothing uses this outside of VacuumFields right now. I'd propose a minor version bump, not a full major release.