Mark OLESEN authored Apr 12, 2021 and Andrew Heather committed Apr 19, 2021

- wrap command-line retrieval of fileName with an implicit validate.

  Instead of this:
      fileName input(args[1]);
      fileName other(args["someopt"]);

  Now use this:
      auto input = args.get<fileName>(1);
      auto other = args.get<fileName>("someopt");

  which adds a fileName::validate on the inputs

  Because of how it is implemented, it will automatically also apply
  to argList getOrDefault<fileName>, readIfPresent<fileName> etc.

- adjust fileName::validate and clean to handle backslash conversion.
  This makes it easier to ensure that path names arising from MS-Windows
  are consistently handled internally.

- dictionarySearch: now check for initial '/' directly instead of
  relying on fileName isAbsolute(), which now does more things

BREAKING: remove fileName::clean() const method

- relying on const/non-const to control the behaviour (inplace change
  or return a copy) is too fragile and the const version was...