MaxLengthStream don't interpolate any position until we know the destination completely

Only build a movement string for the values we know the position of
initialize unknown positions to Unknown (rather than Zero)
Added get to check for fully known position
issue: MatterHackers/MCCentral#4681
Export making M114 assumptions about Z

MatterControlLib/PrinterCommunication/Io/FeedRateMultiplyerStream.cs

@@ -46,7 +46,7 @@ namespace MatterHackers.MatterControl.PrinterCommunication.Io
 
 		public override void SetPrinterPosition(PrinterMove position)
 		{
-			this.LastDestination = position;
+			this.LastDestination.CopyKnowSettings(position);
 			internalStream.SetPrinterPosition(this.LastDestination);
 		}