CaloUtils + CxxUtils + xAODCore + xAODTrackingCnv: Relocate FloatCompressor from xAODCore to CxxUtils

This MR relocates xAOD::FloatCompressor from xAODCore to CxxUtils and it's now called CxxUtils::FloatCompressor. The rationale behind this update is to allow it to be used directly in AthContainers, which is a package xAODCore depends on. All existing clients are migrated.

I already bugged @akraszna and @ssnyder about this via email 😄 Let me keep is as WIP so that they have a chance to object if they think this is not a good idea for whatever reason, although I think it's a fairly uncontroversial move.

Edit: OK, let's un-WIP this.