Add new graph output format to the tree document generator

The "graph" is the data part for genealogytree without the boilerplate
code around it that makes up a full LaTeX file. This format is useful
for people who have their own (more sophisticated) LaTeX file with
custom styles and so on.
The implementation is pretty straight-forward: Just don't output the
LaTeX code.

gramps/gen/plug/docgen/treedoc.py

@@ -586,6 +586,47 @@ class TreeDocBase(BaseDoc, TreeDoc):
         self.write_end()
 
 
+#------------------------------------------------------------------------------
+#
+# TreeGraphDoc
+#
+#------------------------------------------------------------------------------
+class TreeGraphDoc(TreeDocBase):
+    """
+    TreeGraphDoc implementation that generates a .graph file.
+    """
+
+    def write_start(self):
+        """
+        Write the start of the document - nothing for a graph file.
+        """
+        pass
+
+    def start_tree(self, option_list):
+        """
+        Write the start of a tree - nothing for a graph file.
+        """
+        pass
+
+    def end_tree(self):
+        """
+        Write the end of a tree - nothing for a graph file.
+        """
+        pass
+
+    def write_end(self):
+        """
+        Write the end of the document - nothing for a graph file.
+        """
+        pass
+
+    def close(self):
+        """ Implements TreeDocBase.close() """
+        TreeDocBase.close(self)
+
+        with open(self._filename, 'w', encoding='utf-8') as texfile:
+            texfile.write(self._tex.getvalue())
+
 #------------------------------------------------------------------------------
 #
 # TreeTexDoc
@@ -653,6 +694,11 @@ if _LATEX_FOUND:
                  'mime' : "application/pdf",
                  'class': TreePdfDoc}]
 
+FORMATS += [{'type' : "graph",
+             'ext'  : "graph",
+             'descr': _("Graph File for genealogytree"),
+             'class': TreeGraphDoc}]
+
 FORMATS += [{'type' : "tex",
              'ext'  : "tex",
              'descr': _("LaTeX File"),