`TimeHistory` cannot gather the same field from different locations

It’s possible for TimeHistory to gather multiple sources of information into one unique file.
But it’s not possible for the TimeHistory/PackCollection pair to extract the same field on multiple locations and save them into the same file.
Consider the PoroElastic_Mandel test case.
For convenience, generate the cubic mesh with two different cell blocks (here cb1 and cb2):

<Mesh>
 <InternalMesh
  name="mesh1"
  elementTypes="{ C3D8 }"
  xCoords="{ 0.0, 0.5, 1.0 }"
  yCoords="{ 0.0, 0.1 }"
  zCoords="{ 0.0, 1.0 }"
  nx="{ 10, 10 }"
  ny="{ 1 }"
  nz="{ 20 }"
  cellBlockNames="{ cb1, cb2 }"/>
</Mesh>

and accordingly

<ElementRegions>
 <CellElementRegion
  name="Domain"
  cellBlocks="{ cb1, cb2 }"
  materialList="{ shale, water }"/>
</ElementRegions>

Then create/update the Tasks

<PackCollection
 name="pressureCollection"
 objectPath="ElementRegions/Domain/cb1"
 fieldName="pressure"/>
<PackCollection
 name="pressureCollection2"
 objectPath="ElementRegions/Domain/cb2"
 fieldName="pressure"/>

and the TimeHistory

<TimeHistory
 name="pressureHistoryOutput"
 sources="{/Tasks/pressureCollection, /Tasks/pressureCollection2}"
 filename="pressure_history"/>

Do not forget to trigger

<PeriodicEvent
 name="pressureCollection" 
 target="/Tasks/pressureCollection"/>
<PeriodicEvent
 name="pressureCollection2"
 target="/Tasks/pressureCollection2"/>
<PeriodicEvent
 name="pressureOutput"
 targetExactTimestep="0"
 target="/Outputs/pressureHistoryOutput"/>

The case described does not work as TimeHistory does not know how to “merge” the same field into the same file.

Additionally, we can provide one set, but if this set is spread over two sub regions, then the process fails since they can't be merged.

Start with the TimeHistory and PackCollection handling described in the PoroElastic_Mandel example, reproducing the configuration with cb1 and cb2. Done means pressure from both locations, including fields split across subregions, can be merged into one pressure_history file.