use CachedValue::store() for the covariance/position setters (ATLASRECTS-4891)

In the setPosition, setCovariance methods , instead of using reset, which has the effect of the cached values being initialized later on based on the stored values in the xAOD vectors (floats), use store directly with the matrices (double) as arguments.

It might relate with some of the numerical differences here ATLASRECTS-4891

Event/xAOD/xAODTracking/Root/Vertex_v1.cxx

@@ -99,7 +99,7 @@ namespace xAOD {
       setY( position( 1 ) );
       setZ( position( 2 ) );
       // Reset the cache
-      m_position.reset();
+      m_position.store(position);
       return;
    }
 
@@ -124,7 +124,7 @@ namespace xAOD {
 
      // Set the persistent variable:
      setCovariance( vec );
-     m_covariance.reset();
+     m_covariance.store(cov);
      return;
    }