Add efficient hasTag check to Package.

Avoids copying the tags set each time to check for presence / absence
of a particular tag.

simgear/package/Package.cxx

@@ -268,6 +268,11 @@ string_set Package::tags() const
 {
     return m_tags;
 }
+    
+bool Package::hasTag(const std::string& tag) const
+{
+    return m_tags.find(tag) != m_tags.end();
+}
 
 SGPropertyNode* Package::properties() const
 {

simgear/package/Package.hxx

@@ -142,6 +142,13 @@ public:
 
     string_set tags() const;
 
+    /**
+     * @brief hasTag - efficently check if a tag is defined or not
+     * @param tag
+     * @return
+     */
+    bool hasTag(const std::string& tag) const;
+    
     /**
      * download URLs for the package
      */