EAMxx: store fields in a FM inside atm proc class

While working on another feature, it hit me that we could reduce the infrastructure code in `AtmosphereProcess` by quite a bit if we stored the fields inside a `FieldManager`, as that class already has all the logic for retrieving fields. We may need to upgrade the FM class a bit, like adding the ability of adding a pre-built `FieldGroup` (similar to its `add_field` method), but that should not be hard.