fix: BinUtility was auto-convertible from Transform3 (#3691)

This was visible when trying to do

```cpp
std::cout << Transform3{};
```

which would invoke BinUtility's output stream operator, because that wasn't a hidden `friend`, and BinUtility was auto-convertible.

Core/include/Acts/Utilities/BinUtility.hpp

@@ -47,7 +47,7 @@ class BinUtility {
   /// Constructor with only a Transform3
   ///
   /// @param tForm is the local to global transform
-  BinUtility(const Transform3& tForm)
+  explicit BinUtility(const Transform3& tForm)
       : m_binningData(), m_transform(tForm), m_itransform(tForm.inverse()) {
     m_binningData.reserve(3);
   }
@@ -317,13 +317,15 @@ class BinUtility {
     return ss.str();
   }
 
+  /// Overload of << operator for std::ostream for debug output
+  friend std::ostream& operator<<(std::ostream& sl, const BinUtility& bgen) {
+    return bgen.toStream(sl);
+  }
+
  private:
   std::vector<BinningData> m_binningData;  /// vector of BinningData
   Transform3 m_transform;                  /// shared transform
   Transform3 m_itransform;                 /// unique inverse transform
 };
 
-/// Overload of << operator for std::ostream for debug output
-std::ostream& operator<<(std::ostream& sl, const BinUtility& bgen);
-
 }  // namespace Acts

Core/src/Utilities/CMakeLists.txt

@@ -2,7 +2,6 @@ target_sources(
     ActsCore
     PRIVATE
         AnnealingUtility.cpp
-        BinUtility.cpp
         Logger.cpp
         SpacePointUtility.cpp
         TrackHelpers.cpp