[ET Trac] #2948: HydroBaseX: Missing HydroBase features (Aphi GF, conditional storage, split init)

[Maxwell Rizzo]

#2948: HydroBaseX: Missing HydroBase features (Aphi GF, conditional storage, split init)

 Reporter: Maxwell Rizzo
   Status: new
Milestone: 
  Version: 
     Type: bug
 Priority: minor
Component: CarpetX

Current HydroBaseX has a single routine that zeros all the variables it declares, along with a single parameter controlling whether this routine runs: `initial_hydro`. This parameter was used/extended by initial data thorns when they provide hydro quantities.

Following the legacy Carpet/HydroBase convention, `initial_hydro` should only control the minimal hydro fields, with other GFs accompanied by corresponding `initial_*` parameters and routines. The current implementation works, but is awkward for initial hydro data thorns to integrate with: they can either avoid using `initial_hydro` themselves and let HydroBaseX zero all fields and place their data ontop of that, or they can manually zero the fields they are not providing, which is unnecessary when HydroBaseX should be able to do it. 

Additionally, HydroBaseX is missing the `Aphi` GF. This is the electric potential, and adding it would round out the EM support alongside `Bvec` and `Avec`.

Lastly, modularizing this initialization should allow conditional storage to be restored, storing optional hydro fields only when the corresponding `initial_*` parameter is not `"none"`, following HydroBase. This should also provide a minor memory/performance improvement.

Addressed by already open PR: [#385](https://github.com/EinsteinToolkit/CarpetX/pull/385)

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2948/hydrobasex-missing-hydrobase-features-aphi
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