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/TAlignment/include/TAlignment/AlElementHistos.h

@@ -14,7 +14,6 @@
 
 #include "Gaudi/Accumulators/Histogram.h"
 #include "TAlignment/AlignmentElement.h"
-#include "boost/lexical_cast.hpp"
 
 namespace LHCb::Alignment {
 
@@ -37,29 +36,29 @@ namespace LHCb::Alignment {
     AlElementHistos( const OWNER* owner, const Alignment::Element& elem, unsigned int numiter )
         : m_name{elem.name()}
         , m_index{elem.index()}
-        , m_dir{"element" + boost::lexical_cast<std::string>( elem.index() ) + "/"}
+        , m_dir{"element" + std::to_string( elem.index() ) + "/"}
         , m_resmax{elem.histoResidualMax()}
         , m_resHisto{owner,
-                     m_dir + boost::lexical_cast<std::string>( 1000u ),
+                     m_dir + std::to_string( 1000u ),
                      "Residual vs iteration for " + m_name,
                      {numiter, -0.5, numiter - 0.5},
                      {100, -m_resmax, +m_resmax}}
         , m_unbiasedResHisto{owner,
-                             m_dir + boost::lexical_cast<std::string>( 1001u ),
+                             m_dir + std::to_string( 1001u ),
                              "Unbiased Residual vs iteration for " + m_name,
                              {numiter, -0.5, numiter - 0.5},
                              {100, -m_resmax, +m_resmax}}
         , m_pullHisto{owner,
-                      m_dir + boost::lexical_cast<std::string>( 2000u ),
+                      m_dir + std::to_string( 2000u ),
                       "Pull vs iteration for " + m_name,
                       {numiter, -0.5, numiter - 0.5},
                       {100, -5.0, 5.0}}
         , m_nHitsHisto{owner,
-                       m_dir + boost::lexical_cast<std::string>( 3000u ),
+                       m_dir + std::to_string( 3000u ),
                        "Number of hits vs iteration for " + m_name,
                        {numiter, -0.5, numiter - 0.5}}
         , m_autoCorrHisto{owner,
-                          m_dir + boost::lexical_cast<std::string>( 4000u ),
+                          m_dir + std::to_string( 4000u ),
                           "hit residual scale factor in " + m_name + "  vs iteration ",
                           {numiter, -0.5, numiter - 0.5},
                           {100, -1, 1}}