Feature #800

- Added feature #800, adding a new command 'columns' that lists all the columns
  available for custom reports, and includes their formatting options (thanks
  to T. Charles Yun).

src/Date.cpp

@@ -152,6 +152,12 @@ std::string Date::toISO ()
   return iso.str ();
 }
 
+////////////////////////////////////////////////////////////////////////////////
+double Date::toJulian ()
+{
+  return (mT / 86400.0) + 2440587.5;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 void Date::toEpoch (time_t& epoch)
 {