Cleanup field map classes and remove slow field map
For many field map types there are two versions: a fast (marked with _fast in the file name) and a slow version. For a long time the fast version is default and I don't think that anyone is using the slower ones. We should get rid of the slow versions, e.g. the files
src/Classic/Fields/Astra1DDynamic.h
src/Classic/Fields/Astra1DMagnetoStatic.h
src/Classic/Fields/Astra1DElectroStatic.h
src/Classic/Fields/FM1DDynamic.h
src/Classic/Fields/FM1DMagnetoStatic.h
src/Classic/Fields/FM1DElectroStatic.h