Use event-local memory pool when reading RawEvent

This uses the tools of !2270 (merged) when reading raw events from MDF.

There is a slight complication thanks to a bug in ROOT: https://sft.its.cern.ch/jira/browse/ROOT-10526 A fix has been proposed, but I haven't been able to test it yet. The fix should make it to the 6.20 release, and presumably will be available via the LCG dev3 and/or dev4 slots before then. The effect is that ROOT I/O of LHCb::RawEvent is not possible with this MR and without the ROOT fix.

I added a unit test to this MR that exposes the problem (it segfaults/fails).

cc: @sponce @frankm

DAQ/MDF/MDF/Buffer.h

@@ -24,19 +24,17 @@ namespace LHCb::MDF {
   /**
    * This class represents a single event in the event buffer
    * Upon creation, it only gets a raw buffer and a size.
-   * Banks are then decoded from the buffer and added to the m_event when
+   * Banks are then decoded from the buffer and added to a RawEvent when
    * calling get on the Buffer object
    */
   class MDFEvent {
   public:
     MDFEvent() = default;
     MDFEvent( LHCb::span<std::byte> data ) : m_data( data ) {}
-    LHCb::RawEvent& event() { return m_event; }
-    std::byte*      data() { return m_data.data(); }
-    unsigned int    size() { return m_data.size(); }
+    std::byte*   data() { return m_data.data(); }
+    unsigned int size() { return m_data.size(); }
 
   private:
-    LHCb::RawEvent        m_event;
     LHCb::span<std::byte> m_data;
   };
 
@@ -57,10 +55,10 @@ namespace LHCb::MDF {
     static void setReservedNumberOfBanks( unsigned int nbBanksReserve ) { s_nbBanksReserve = nbBanksReserve; }
     /**
      * get the next event in the Buffer. This method is thread safe
-     * and garantees to return evry single event exactly once
+     * and guarantees to return every single event exactly once
      * @returns the event or nothing when the buffer is empty
      */
-    std::optional<LHCb::RawEvent> get();
+    std::optional<LHCb::RawEvent> get( EventContext const& evtCtx );
 
   private:
     /// vector of RawEvents, pointing to the Rawbanks in rawBuffer