Aquifer flux unit in the log at each timestep.

[MelReyCG]

Describe the bug
In the following log line, the flux units are not very clear, and might be wrong.
FlowSolverBase compositionalMultiphaseFlow (SimuDeck_aquifer_pression_meme.xml, l.28): at time 100s, the <Aquifer> boundary condition 'aquifer1' produces a flux of -0.6181975187076816 kg (or moles if useMass=0).
It may be a volume unit that needs to be determined.
The message appears for each Aquifer at each timestep, if logLevel >= 1.

Expected behavior
A clearer unit expression of the flux.

Side note
We are currently investigating this. The message could be edited in one of my current PRs.