DOC: Adjust initialization rule for padding data, smart pointers, etc.

Excluded padding data, `std::unique_ptr`, `itk::SmartPointer` and low level
utility classes from the guideline that says that "all member variables must be
initialized when they are declared".

`std::unique_ptr` and `itk::SmartPointer` are excluded because of some GCC
compile errors, which were addressed in ITK by Simon Rit:

 - pull request InsightSoftwareConsortium/ITK#3877
   commit InsightSoftwareConsortium/ITK@eac289d
   "COMP: Remove in-class {} member initializers of unique_ptr"
 - pull request InsightSoftwareConsortium/ITK#3927
   commit InsightSoftwareConsortium/ITK@f5f8367
  "COMP: Remove in class init of SmartPointer of forward declaration"

Author: N-Dekker

SoftwareGuide/Latex/Appendices/CodingStyleGuide.tex

@@ -1621,8 +1621,23 @@ \section{Initialization and Assignment}
 \end{minted}
 \normalsize
 
-Smart pointers need not to be initialized, since they initialize themselves
-to the \code{null} pointer, so they are the sole exception to the above rule.
+Exceptions to this guideline:
+
+Private data members that are just there to add padding bytes between other
+(member) variables (typically to avoid false sharing in the context of
+multi-threading) should not be initialized. Such a padding data member is
+typically declared as a C-style array of a (possibly unsigned) character type.
+
+A data member that is declared as \code{std::unique\_ptr<T>} or
+\code{itk::SmartPointer<T>} should not have an empty \code{\{\}} initializer at
+its declaration, at least for now, because of a GCC issue (prior to GCC release
+9.2), which would cause compilation errors when the type \code{T} is an
+incomplete (forward declared) class type.
+
+Another exception is made for low level utility classes for which data member
+initialization would cause a significant performance penalty. This is why for
+example \code{FixedArray::m\_InternalArray} and \code{Index::m\_InternalArray}
+do not have a default member initializer.
 
 Note that all numeric data members must be initialized using the appropriate
 ITK's \code{NumericTraits} static method.