Update to allow Alignment to run multithreaded

Modifications to allow running multithreaded:

• rewrote AlElementHistos to use Accumulator::Histogram
• changed from mutex to shared_mutex to profit from readonly locks
• create AlEquations object in each event call on the stack and add at end of event to minimize number of write locks
• AlignAlgorithm no longer derives from GaudiHistoAlg
• update gaudiiter to support running multi-threaded (works on MDF files only)

This addresses most of what is discussed in #30

This also fixes a crash in a test on clang that was due to a bug in the IO of the Al::Equations object.

@graven, @jcobbled

Alignment/AlignKernel/include/AlignKernel/AlEquations.h

@@ -33,8 +33,8 @@ namespace LHCb::Alignment {
     OffDiagonalData() : m_matrix(), m_numTracks( 0 ), m_numHits( 0 ) {}
     Gaudi::Matrix6x6&       matrix() { return m_matrix; }
     const Gaudi::Matrix6x6& matrix() const { return m_matrix; }
-    void                    addTrack() { ++m_numTracks; }
     size_t                  numTracks() const { return m_numTracks; }
+    auto&                   numTracks() { return m_numTracks; }
     size_t                  numHits() const { return m_numHits; }
     template <class T>
     void add( const T& m ) {
@@ -67,6 +67,7 @@ namespace LHCb::Alignment {
       m_alpha      = alpha;
       m_alphaIsSet = true;
     }
+    auto                  alphaIsSet() const { return m_alphaIsSet; }
     void                  setAlignFrame( const Gaudi::Vector6& alignframe ) { m_alignframe = alignframe; }
     const Gaudi::Vector6& alignFrame() const { return m_alignframe; }
 
@@ -88,7 +89,7 @@ namespace LHCb::Alignment {
       return m_numHits > 0 ? Gaudi::XYZPoint( m_sumX / m_numHits, m_sumY / m_numHits, m_sumZ / m_numHits )
                            : Gaudi::XYZPoint();
     }
-    void addTrack() { ++m_numTracks; }
+    auto& numTracks() { return m_numTracks; }
 
     Gaudi::Vector6       m_alignframe;        // parameters of the alignment frame
     Gaudi::Vector6       m_alpha;             // the set of parameters at which the derivatives are computed
@@ -161,7 +162,10 @@ namespace LHCb::Alignment {
     void readFromFile( const char* filename );
     void writeToBuffer( std::ofstream& buffer ) const;
     void readFromBuffer( std::ifstream& buffer );
-    void add( const Equations& );
+
+    /// Add the data from another Equations object. If correctDeltaAlpha is true, it will check that the other
+    /// object was computed around the same parameter set, and if not, correct for the difference.
+    void add( const Equations&, bool correctDeltaAlpha = false );
 
     std::ostream& fillStream( std::ostream& os ) const;