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/AlFileIO.h

@@ -22,14 +22,14 @@ namespace LHCb::Alignment {
     template <typename T>
     std::ofstream& operator<<( std::ofstream& file, const T& object ) {
       // printf("Trying to write object with size: %u\n",sizeof( object )) ;
-      //static_assert( std::is_trivially_copyable_v<T> );
+      // static_assert( std::is_trivially_copyable_v<T> );
       file.write( reinterpret_cast<const char*>( &object ), sizeof( object ) );
       return file;
     }
     template <typename T>
     std::ifstream& operator>>( std::ifstream& file, T& object ) {
       // printf("Trying to read object with size: %u\n",sizeof( object )) ;
-      //static_assert( std::is_trivially_copyable_v<T> );
+      // static_assert( std::is_trivially_copyable_v<T> );
       file.read( reinterpret_cast<char*>( &object ), sizeof( object ) );
       return file;
     }
@@ -40,7 +40,7 @@ namespace LHCb::Alignment {
       size_t n = amap.size();
       // printf("Trying to write map with size: %d\n",int(n)) ;
       file << n;
-      for ( const auto& [f,s] : amap ) file << f << s;
+      for ( const auto& [f, s] : amap ) file << f << s;
       return file;
     }
     template <typename Key, typename T, typename Compare, typename Allocator>