Clean up HltLumiSummaryDecoder

The following discussion from !3518 (merged) should be addressed:

• @graven started a discussion:

  I don't like large, repetitive blocks of code where the same thing happens, but just with different data -- so how about defining the data, and then looping over it, eg. by first defining a tiny struct to hold the data:

    struct Field {
       const char* name;
       LHCb::LumiSummaryOffsets::V2::counterOffsets offset;
       LHCb::LumiSummaryOffsets::V2::counterOffsets size;     
    }; 

  and then define the data as:

    constexpr auto fields = std::array{ Field{ "encodingKey", LHCb::LumiSummaryOffsets::V2::encodingKeyOffset, LHCb::LumiSummaryOffsets::V2::encodingKeySize },
                                        Field{ "T0Low",       LHCb::LumiSummaryOffsets::V2::T0LowOffset,       LHCb::LumiSummaryOffsets::V2::T0LowSize },
                                        Field{ "T0High",      LHCb::LumiSummaryOffsets::V2::T0HighOffset,      LHCb::LumiSummaryOffsets::V2::T0HighSize },
                                        ...
    };

  and then finally do:

      for ( const auto& [name, offset, size] : fields ) hltLumiSummary.addInfo( name, getField( offset, size, ibank->data() ) );