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

@@ -157,14 +157,14 @@ namespace LHCb::Alignment {
     size_t      totalTrackNumDofs() const { return totalNumDofs() - m_totalVertexNumDofs; }
     size_t      numExternalHits() const { return m_numExternalHits; }
 
-    void writeToFile( const char* filename ) const;
-    void readFromFile( const char* filename );
-    void writeToBuffer( std::ofstream& buffer ) const;
-    void readFromBuffer( std::ifstream& buffer );
+    std::tuple<size_t, long>                        writeToFile( const char* filename ) const;
+    std::tuple<StatusCode, std::string>             readFromFile( const char* filename );
+    void                                            writeToBuffer( std::ofstream& buffer ) const;
+    void                                            readFromBuffer( std::ifstream& buffer );
 
     /// 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::map<std::string, std::vector<std::string>> add( const Equations&, bool correctDeltaAlpha = false );
 
     std::ostream& fillStream( std::ostream& os, int verbosity = 0 ) const;