ENH: additional IndirectList static methods

- uniq() : creates an IndirectList with duplicated entries
  filtered out

- subset() : creates an IndirectList with positions that satisfy
  a condition predicate.

- subset_if() : creates an IndirectList with values that satisfy a
  given predicate.

  An indirect subset will be cheaper than creating a subset copy
  of the original data, and also allows modification.

STYLE: combine UIndirectList.H into UIndirectList.H (reduce file clutter)

src/sampling/sampledSet/sampledSets/sampledSets.C

@@ -34,7 +34,7 @@ License
 #include "mapPolyMesh.H"
 #include "IOmanip.H"
 #include "IOobjectList.H"
-#include "UIndirectList.H"
+#include "IndirectList.H"
 #include "ListOps.H"
 #include "addToRunTimeSelectionTable.H"
 

src/sampling/sampledSurface/sampledSurfaces/sampledSurfaces.C

@@ -35,7 +35,7 @@ License
 #include "HashOps.H"
 #include "ListOps.H"
 #include "Time.H"
-#include "UIndirectList.H"
+#include "IndirectList.H"
 #include "addToRunTimeSelectionTable.H"
 
 // * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //