Add `external_field` argument to `QuadraticFlux` and other normal field objectives

[dpanici]

If I want to say optimize a modular coilset in the presence of a background ToroidalField, currently we would have to do so with a SumMagneticField object, passing in the coils and the TF. however, our coil objectives cannot handle a SumMagneticField object so we would be unable to penalize coil coil distance or curvature or anything like that.

If we add in a external_field argument to these objectives that can take in a static external field, we would avoid these issues.

A longer term fix would be to get the coil objectives to work with objects where the field is not given solely by coilsets.