feat: Change molarity concentration to molality concentration

[frankfeifan]

This PR changes convention of the concentration fed into HPCReact from molarity [mol/m3 of solution] or [mol/L of solution] to molality [mol/kg of solvent] using a factor called solventMassPerSolutionVolume (not solventDensity anymore) to be consistent with convention.

This is a preparatory step for introducing the activity model. So now, HPCReact is treated as a strictly molality-based library, and every conversion to a volumetric basis happens on the GEOS side through a single factor.

Specific changes include:

• Switched the concentration passed to HPCReact from molarity to molality, converting back at every caller: accumulation, advective and diffusive flux, Dirichlet and source terms, and their thermal counterparts.

• Introduced solventMassPerSolutionVolume [kg/m3] as an optional input on the reactive fluid model (default 1000) to serve as that conversion factor. For now, it is prescribed and kept constant, but ideally, we want to make it a function of P, T, and chemical composition. Specific implementation of methods and where these updates will be launched (GEOS or HPCReact) needs more discussions and deserves another PR.

• Sourced the solventMassPerSolutionVolume from the solver's single reactive fluid model rather than a per-subregion scan, and turned the previously silent single-fluid-model assumption into an explicit input error, ensuring a single fluid model defined across all regions for a uniform dof layout.

• Restored the finite-difference Jacobian test to dt = 1.0, since the reduction to 0.01 was a workaround for the inflated reaction term.

• Rebaseline

Note: No HPCReact change is required, as the conversion factor lives in GEOS now tentatively.

[jafranc]

Looks great 👍

[tjb-ltk]

A future discussion could consider posing this as multicomponent single phase option where specializations could be this could be this option or something like tracers .. not sure about the implications when starting with a num component generalization...

[frankfeifan]

A future discussion could consider posing this as multicomponent single phase option where specializations could be this could be this option or something like tracers .. not sure about the implications when starting with a num component generalization...

Yeah. That's a good point. We may need a higher-level generic solver base for such multi-component single phase option (can inherit from SinglePhaseBase as I did for this reactive transport solver).

[OmarDuran]

Double check if we actually need this.

Suggested change

	using namespace hpcReact::bulkGeneric;
	using namespace hpcReact::geochemistry;
	using namespace hpcReact::ChainGeneric;
	using namespace hpcReact::MoMasBenchmark;

[OmarDuran]

@frankfeifan thank you for taking care of computing actual mass balance.

I might be mistaken, but only with constant ρs, the conversions are fully correct. I’m unsure whether they remain consistent for a compressible fluid.

I also refactored a few files to accommodate these changes. See the patch below; if it makes sense, feel free to apply it.

molality_solventMassFraction.patch