Allow extends from empty base-class and define empty class.

Closes modelica#3320

chapters/classes.tex

@@ -843,7 +843,7 @@ \subsection{Short Class Definitions}\label{short-class-definitions}
 
 \subsection{Combining Base Classes and Other Elements}\label{restriction-on-combining-base-classes-and-other-elements}\label{combining-base-classes-and-other-elements}
 
-It is not legal to combine equations, algorithms, components, base classes, or protected elements with an extends from an array class, a class with non-empty \lstinline[language=grammar]!base-prefix!, a \firstuse{simple type} (\lstinline!Real!, \lstinline!Boolean!, \lstinline!Integer!, \lstinline!String! and enumeration types), or any class transitively extending from an array class, a class with non-empty \lstinline[language=grammar]!base-prefix!, or a simple type.
+It is not legal to combine equations, algorithms, components, non-empty base classes (see \cref{empty-class}), or protected elements with an extends from an array class, a class with non-empty \lstinline[language=grammar]!base-prefix!, a \firstuse{simple type} (\lstinline!Real!, \lstinline!Boolean!, \lstinline!Integer!, \lstinline!String! and enumeration types), or any class transitively extending from an array class, a class with non-empty \lstinline[language=grammar]!base-prefix!, or a simple type.
 
 \begin{example}
 \begin{lstlisting}[language=modelica]
@@ -893,6 +893,11 @@ \subsection{Local Class Definitions -- Nested Classes}\label{local-class-definit
 The variables \lstinline!v1! and \lstinline!v2! are instances of this local class and thus have a nominal value of 1000.
 \end{example}
 
+\subsection{Empty Class}\label{empty-class}
+An empty class is a class without equations, algorithms, and components, and where any base-classes are themselves empty.
+
+They may contain graphics - and can be used more freely as base-classes than other classes.
+
 \section{Specialized Classes}\label{specialized-classes}
 
 Specialized kinds of classes\index{specialized class} (earlier known as \emph{restricted classes}\index{restricted class|see{specialized class}})

chapters/functions.tex

@@ -2824,7 +2824,7 @@ \subsection{External Objects}\label{external-objects}
 \item
   An external object class shall be of the specialized class {\lstinline!class!}.
   \begin{nonnormative}
-  This is the only use of {\lstinline!class!}.
+  This is the only use of {\lstinline!class!} for a non-empty class, \cref{empty-class}.
   \end{nonnormative}
 \item
   Classes derived from {\lstinline!ExternalObject!} can neither be used in an {\lstinline!extends!}-clause nor in a short class definition.