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

@@ -17,7 +17,7 @@
 
 // FIXME: temporarily disable a non-understood warning on gcc11
 #ifndef __clang__
-#pragma GCC diagnostic ignored "-Wsubobject-linkage"
+#  pragma GCC diagnostic ignored "-Wsubobject-linkage"
 #endif
 
 namespace LHCb::Alignment {