distributions: Generalise usage

The distributions have been extended in various ways to permit usage in
a greater variety of situations...

The distributions now have write methods which allow a distribution to
be written into a field file for restart, therby permitting their usage
in boundary conditions and similar. Their read methods now also support
dimension-checked unit conversions for all their parameters.

An additional selector has been added that allows a distribution to be
re-constructed with a different sample size exponent.

The distributions now own their random generator, thereby simplifying
their usage and preventing the need for a (potentially dangling)
reference member. This makes sense now as the random generators do not
rely on global state; individual sub-models can and should own their own
random generator and manage its initialisation and restart. This
principle should be extended to other parts of the code in future.

applications/utilities/postProcessing/miscellaneous/pdfPlot/createFields.H

@@ -24,17 +24,7 @@
 
     mkDir(pdfPath);
 
-    randomGenerator rndGen(label(0));
-
-    autoPtr<distribution> p
-    (
-        distribution::New
-        (
-            pdfDictionary,
-            rndGen,
-            0
-        )
-    );
+    autoPtr<distribution> p(distribution::New(unitAny, pdfDictionary, 0, 0));
 
     const scalar xMin = p->min();
     const scalar xMax = p->max();