Streamline MagneticField grid reader

• Streamline magnetic field grid reader and improve error checking
• prefer std::unique_ptr over std::shared_ptr

---

Validated by

• Core Software
• RTA
• Simulation

Core/include/Core/MagneticFieldExtension.h

@@ -41,7 +41,7 @@ namespace LHCb::Magnet {
       bool                               useRealMap{ true };
       bool                               isDown{ true };
       double                             signedRelativeCurrent{ 1.0 };
-      std::shared_ptr<MagneticFieldGrid> grid;
+      std::unique_ptr<MagneticFieldGrid> grid;
     };
 
   public: