ENH: allow passing of comparator to sortToc methods

- this increases the flexibility of the interface

- Add stringOps 'natural' string sorting comparison.
  Digits are sorted in their natural order, which means that
      (file10.txt file05.txt file2.txt)
  are sorted as
      (file2.txt file05.txt file10.txt)

STYLE: consistent naming of template parameters for comparators

  - Compare for normal binary predicates
  - ListComparePredicate for list compare binary predicates

src/OpenFOAM/primitives/strings/stringOps/stringOps.H

@@ -41,6 +41,7 @@ SourceFiles
 #include "word.H"
 #include "dictionary.H"
 #include "HashTable.H"
+#include "stringOpsSort.H"
 
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