Replace deprecated mark-as IDF syntax

Replaces the old mark-as syntax with the newer
and not deprecated is= syntax to avoid warnings
on loading the IDF.

instrument/PEARL_Definition_new_lowangle.xml

@@ -159,7 +159,7 @@
 </type>
 
 <type name="monitors defined by Chris">
-<component type="monitor" mark-as="monitor">
+<component type="monitor">
   <location  r="1.62155697" t="180" p="0" name="upstream_monitor_det1" />
   <location  r="1.56719915" t="180" p="0" name="upstream_monitor_det2" />
   <location  r="0.74473986" t="180" p="0" name="upstream_monitor_det3" />
@@ -178,7 +178,7 @@
 </type>
 
 <!-- detector and monitor types -->
-<type name="monitor" is="detector">
+<type name="monitor" is="monitor">
   <properties/>
   <cylinder id="some-shape">
  <centre-of-bottom-base r="0.0" t="0.0" p="0.0" />