Add writing and reading of RecSummary

Requires LHCb!3671 (merged) and Rec!3007 (merged) and !1649 (merged)

Needed for Rec#366 (closed)

This MR adds the creation and writing of the RecSummary to HLT2. The RecSummary is a collection of multiplicities from all detectors or several reconstruction steps. As the reconstruction is not persisted after Turbo or Sprucing, the RecSummary needs to be created in HLT2, otherwise the information is lost.

• Added RecSummary as output of the reconstruction in hlt2_global_reco.py.
• The creation of the RecSummary is foreced by the Hlt2Line object. The RecSummary is treated similarly to PVs, every line adds it to the required outputs.
• Added serialization of RecSummary to persistence.
• Added basic test of persistence.

The location is forced to be /Event/HLT2/Rec/Summary at the moment.

TODO list:

• Propagate RecSummary through Sprucing.
• Fill more variables.