pcbnew: Re-add missing user units python

When moving user user units out of the global, we missed that Python
calls were using it.  This re-adds a function to retrieve user units
allowing the python scripts to show users dialogs in their preferred
units.

pcbnew/swig/pcbnew_scripting_helpers.cpp

@@ -146,6 +146,15 @@ void UpdateUserInterface()
 }
 
 
+int GetUserUnits()
+{
+    if( s_PcbEditFrame )
+        return s_PcbEditFrame->GetUserUnits();
+
+    return -1;
+}
+
+
 bool IsActionRunning()
 {
     return ACTION_PLUGINS::IsActionRunning();

pcbnew/swig/pcbnew_scripting_helpers.h

@@ -69,6 +69,12 @@ void    WindowZoom( int xl, int yl, int width, int height );
  */
 void UpdateUserInterface();
 
+/**
+ * Returns the currently selected user unit value for the interface
+ * @return 0 = Inches, 1=mm, -1 if the frame isn't set
+ */
+int GetUserUnits();
+
 /**
  * Are we currently in an action plugin?
  */