[units] yet another units fix

Units that can be automatically converted should come last so other
possibilities are tested first. (ok, that is a lazy and fragile
solution...)
A the type of auto conversion ("display" or "code" can be changed only
for conversions that were already using auto mode (eg. deg <-> decideg
will not be auto converted)

conf/units.xml

@@ -1,10 +1,6 @@
 <!-- Table of default units convertion -->
 <!-- used to convert from unit to alt_unit (messages) or code_unit (airframe) -->
 <units>
-  <unit from="deg" to="rad" coef="0.0174532925" auto="code"/>
-  <unit from="rad" to="deg" coef="57.2957795131" auto="display"/>
-  <unit from="deg/s" to="rad/s" coef="0.0174532925" auto="code"/>
-  <unit from="rad/s" to="deg/s" coef="57.2957795131" auto="display"/>
   <unit from="m" to="cm" coef="100."/>
   <unit from="cm" to="m" coef="0.01"/>
   <unit from="m/s" to="cm/s" coef="100."/>
@@ -15,4 +11,8 @@
   <unit from="mm/s" to="m/s" coef="0.001"/>
   <unit from="decideg" to="deg" coef="0.1"/>
   <unit from="deg" to="decideg" coef="10."/>
+  <unit from="deg" to="rad" coef="0.0174532925" auto="code"/>
+  <unit from="rad" to="deg" coef="57.2957795131" auto="display"/>
+  <unit from="deg/s" to="rad/s" coef="0.0174532925" auto="code"/>
+  <unit from="rad/s" to="deg/s" coef="57.2957795131" auto="display"/>
 </units>

sw/lib/ocaml/pprz.ml

@@ -173,9 +173,11 @@ let scale_of_units = fun ?auto from_unit to_unit ->
           (* will raise Xml.No_attribute if not a valid attribute *)
           let f = Xml.attrib u "from"
           and t = Xml.attrib u "to"
-          and a = match auto with
-            | Some a -> a
-            | None -> String.lowercase (ExtXml.attrib_or_default u "auto" "") in
+          and a = try Some (Xml.attrib u "auto") with _ -> None in
+          let a = match auto, a with
+            | Some _, None | None, None -> "" (* No auto conversion *)
+            | Some t, Some _ | None, Some t -> String.lowercase t (* param auto is used before attribute *)
+          in
           if (f = from_unit || a = "display") && (t = to_unit || a = "code") then true else false
         ) (Xml.children units_xml) in
       (* return coef, raise Failure if coef is not a numerical value *)