Add specification for importance factor reporting

The commonly calculated importance factors (DIF, MIF, ...)
can be reported in attributes of variables (basic events).
The approach has been tested with SCRAM
and is flexible for new factors or omitted factors.

Note that importance factor reporting is missing for CCF events
because the general representation of them
is not yet accepted into the report layer.
It should be trivial to add CCF event importance factor reporting
(if not cumbersome, though)
after the initial formatting is established.

Author: rakhimov

en_us.ascii.add.spl

@@ -104,3 +104,10 @@ minipage
 tabularcolumns
 textwidth
 tfrac
+Birnbaum
+CIF
+DIF
+Fussel
+MIF
+RRW
+Vesely

mef/report_layer.rst

@@ -85,6 +85,7 @@ The following three categories are so frequent
 that is it worth to normalize the way they are stored into XML files.
 
 - Minimal cut sets (and prime implicants)
+- Importance factors
 - Statistical measures (with moments)
 - Curves
 
@@ -125,6 +126,46 @@ to carry the relevant information.
         | <not> <basic-event name="identifier" /> </not>
 
 
+Importance factors
+------------------
+
+Importance factors are typically computed for basic events
+with the same configurations as for probability analysis.
+The following importance factors are long-established and commonly calculated:
+
+    - Fussel-Vesely Diagnosis Importance Factor (DIF)
+    - Birnbaum Marginal Importance Factor (MIF)
+    - Critical Importance Factor (CIF)
+    - Risk Reduction Worth (RRW)
+    - Risk Achievement Worth (RAW)
+
+The definitions of these factors can be found in PSA/PRA literature.
+
+.. code-block:: bnf
+    :name: bnf_importance_factors
+    :caption: Backus-Naur form for the XML representation of importance factors
+
+    importance ::=
+        <importance
+            [ name="identifier" ]
+            [ description="text" ]
+            [ basic-events="integer" ]
+            [ warning="text" ]
+        >
+            importance-event*
+        </importance>
+
+    importance-event ::=
+        <basic-event
+            name="identifier"
+            [ DIF="float" ]
+            [ MIF="float" ]
+            [ CIF="float" ]
+            [ RRW="float" ]
+            [ RAW="float" ]
+        />
+
+
 Statistical measures
 --------------------