Hello,

This MR is hopefully the last one in the "adding tau cells to xAOD" series :-)
We now save the cells of photon shot clusters, which are built from a handful of EM1 cells.
There is almost no xAOD size increase due to shot cells being added to the AllCalo container, because these cells are already part of other EM clusters (tau pi0s) for a large part.
The only extra disk footprint comes from the small container of cluster-to-cell links (0.3 kb/event seen in standalone reconstruction of MC ESD).

The only change to the xAOD output is, as expected:
```
Py:diff-root      WARNING the following variables exist only in the new file !
Py:diff-root      WARNING  - [TauShotClustersAuxDyn.CellLink.m_persKey]
Py:diff-root      WARNING  - [TauShotClustersAuxDyn.CellLink]
Py:diff-root      WARNING  - [TauShotClustersAuxDyn.CellLink.m_persIndex]
Py:diff-root      WARNING  - [TauShotClusters_links]
```

Cheers,
Bertrand