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

@@ -34,7 +34,7 @@ namespace LHCb::Alignment {
     Gaudi::Matrix6x6&       matrix() { return m_matrix; }
     const Gaudi::Matrix6x6& matrix() const { return m_matrix; }
     size_t                  numTracks() const { return m_numTracks; }
-    auto& numTracks() { return m_numTracks; }
+    auto&                   numTracks() { return m_numTracks; }
     size_t                  numHits() const { return m_numHits; }
     template <class T>
     void add( const T& m ) {
@@ -67,7 +67,7 @@ namespace LHCb::Alignment {
       m_alpha      = alpha;
       m_alphaIsSet = true;
     }
-    auto alphaIsSet() const { return m_alphaIsSet ; }
+    auto                  alphaIsSet() const { return m_alphaIsSet; }
     void                  setAlignFrame( const Gaudi::Vector6& alignframe ) { m_alignframe = alignframe; }
     const Gaudi::Vector6& alignFrame() const { return m_alignframe; }