Update German stemmer to the latest standard

Basically, it the German2 variant is recommended now. This feature was present before, but was turned off by default. Now German2 is the default.

Author: Blake-Madden

Changelog.md

@@ -1,7 +1,7 @@
 ## Change Log
 
 ### 2023.1 Release
-- Updated the Swedish stemmer to the latest Snowball standard.
+- Updated the Swedish and German stemmers to the latest Snowball 2.3 standard.
 
 ### 2023 Release
 - Updated Spanish, Russian, Italian, and French stemmers to the latest Snowball standard.

src/german_stem.h

@@ -27,6 +27,16 @@ namespace stemming
 
     @par Algorithm:
 
+    <b>Step 0:</b>
+
+    - Replace ß with ss, ae with ä, oe with ö, ue with ü (unless preceded by q).
+
+        The rules here for ae, oe and ue were
+        added in Snowball 2.3.0, but were previously present as a variant of the
+        algorithm termed"german2". The condition on the replacement of ue prevents
+        the unwanted changing of quelle. Also note that feuer is not modified
+        because the first part of the rule changes it to feUer, so ue is not found.
+
     <b>Step 1:</b>
 
     Search for the longest among the following suffixes:
@@ -70,8 +80,9 @@ namespace stemming
     class german_stem final : public stem<string_typeT>
         {
     public:
-        german_stem() noexcept : m_transliterate_umlauts(false) {}
-        /** @brief Set to true to use the variant algorithm that expands "ä" to "ae", etc...
+        /** @brief Set to @c true (the default) to use the algorithm that expands "ä" to "ae", etc...
+            @details This should only be @c false if preferring to use the German algorithm prior
+                to the Snowball 2.3.0 standard.
             @param transliterate_umlauts Whether to transliterate umlauted vowels.*/
         void should_transliterate_umlauts(const bool transliterate_umlauts)
             { m_transliterate_umlauts = transliterate_umlauts; }
@@ -361,7 +372,7 @@ namespace stemming
                     }
                 }
             }
-        bool m_transliterate_umlauts;
+        bool m_transliterate_umlauts{ true };
         };
     }