For event size analytics

To go with Moore!1356 (merged)

Motivation

Want event size analysis of Spruced events per line. See https://gitlab.cern.ch/lhcb-dpa/project/-/issues/90

How

Exploit the fact that each stream gets its own instance of CombineRawBankViewsToRawEvent.

Add Gaudi::Accumulators::StatCounter to CombineRawBankViewsToRawEvent to give table of the form

EvtSize_<line name>    INFO Number of counters : X
 |    Counter                                      |     #     |    sum | mean/eff^* | rms/err^*  |     min   |     max    |
 | "DstData bank size (bytes)"                     |         X |      Y |         YY |     YYY    |      YYYY |      YYYYY |
 | "Event size (bytes)"                            |         X |      Z |         ZZ |     ZZZ    |      ZZZZ |      ZZZZZ |

Note that by running CombineRawBankViewsToRawEvent in VERBOSE mode - achieved through CombineRawBankViewsToRawEvent.bind(OutputLevel=1) one also has the log of the individual sizes of ALL RawBanks in banks.