ENH: add default "constant" name for fileOperations::findTimes()

- makes fileHandler calling resemble Time more closely

ENH: provide natural_sort less/greater functions

- more succinct than (compare < 0) or (compare > 0).
  The corresponding wrappers for UList renamed as list_less, etc
  but they were only used in unit tests

ENH: handle (-allRegions | -all-regions, ..) directly when retrieving args

- makes handling independent of aliasing order

STYLE: include <string_view> when including <string>

- the standard does not guarantee which headers (if any) actually
  include string_view

src/OpenFOAM/primitives/strings/string/string.H

@@ -54,6 +54,7 @@ SourceFiles
 #include "Hasher.H"
 #include <cstdlib>
 #include <cstring>
+#include <string_view>
 #include <string>
 
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