Add "probability" & "occurrence" in importance report

The idea is borrowed from XFTA.

Author: rakhimov

mef/report_layer.rst

@@ -124,6 +124,11 @@ The following importance factors are long-established and commonly calculated:
 
 The definitions of these factors can be found in PSA/PRA literature.
 
+In addition to the importance factors,
+this report section may contain
+the evaluated probability of an event and
+the number of products it occurs in.
+
 .. literalinclude:: schema/importance_factors.rnc
     :name: schema_importance_factors
     :caption: The RNC schema for the XML representation of importance factors

mef/schema/importance_factors.rnc

@@ -9,6 +9,8 @@ importance =
 importance-event =
   element basic-event {
     name,
+    attribute occurrence { xsd:nonNegativeInteger }?,
+    attribute probability { xsd:double }?,
     attribute DIF { xsd:double }?,
     attribute MIF { xsd:double }?,
     attribute CIF { xsd:double }?,