MAVLinkParam: fix rounding to 7 digits

Rounding to fixed number of decimal places led to over rounding small
numbers and under-rounding big numbers.

ExtLibs/Mavlink/MAVLinkParam.cs

@@ -129,12 +129,21 @@ public double GetValue()
 (double)item.float_value
 0.800000011920929
  */
-                    return Math.Round((double)float_value, 7);
+                    return RoundToSignificantDigits(float_value, 7);
             }
 
             throw new FormatException("invalid type");
         }
 
+        private double RoundToSignificantDigits(double d, int digits)
+        {
+            if (d == 0)
+                return 0;
+
+            double scale = Math.Pow(10, Math.Floor(Math.Log10(Math.Abs(d))) + 1);
+            return scale * Math.Round(d / scale, digits);
+        }
+
         public void SetValue(double input)
         {
             switch (Type)