ENH: Declare converting Point(v) constructors explicit

Prevented implicit conversion of a value to `Point`. Aims to avoid
possible flaws, like the ones addressed by:

Pull request InsightSoftwareConsortium#3225
commit 5b71d63
"BUG: Quick-fix ContourSpatialObject::Update(), LINEAR_INTERPOLATION case"

Pull request InsightSoftwareConsortium#3228
commit 12a501c
"BUG: Fix `SetCenterInObjectSpace` calls in Registration test"

Pull request InsightSoftwareConsortium#3231
commit e4841aa
"STYLE: Clarify estimation ray position `RayCastInterpolateImageFunction`"

Deleted `Point(nullptr_t)`, especially to avoid `PointType p = 0`.

Modules/Core/Common/include/itkPoint.h

@@ -99,12 +99,16 @@ class ITK_TEMPLATE_EXPORT Point : public FixedArray<TCoordRep, VPointDimension>
   Point(const ValueType r[VPointDimension])
     : BaseArray(r)
   {}
-  /** Pass-through constructors for single values */
+
+  /** Prevents copy-initialization from `nullptr`, as well as from `0` (NULL). */
+  Point(std::nullptr_t) = delete;
+
+  /** Explicit constructors for single values */
   template <typename TPointValueType>
-  Point(const TPointValueType & v)
+  explicit Point(const TPointValueType & v)
     : BaseArray(v)
   {}
-  Point(const ValueType & v)
+  explicit Point(const ValueType & v)
     : BaseArray(v)
   {}