Append grep results to avoid overwriting header

Currently, if "keep_header" is true, the header is copied to the output file, but is then overwritten by the grep results.

tools/filters/grep.xml

@@ -17,7 +17,7 @@
     #else
       cat '$input'
     #end if
-    | grep -P -f '$pattern_file' $invert > '$out_file1'
+    | grep -P -f '$pattern_file' $invert >> '$out_file1'
   ]]></command>
   <configfiles>
     <configfile name="pattern_file">$pattern</configfile>