ENH: improve findInstance handling for optional files

- previously would always return "constant" as the instance for
  an optional dir/file that wasn't found.
  However, this meant retesting to screen out false positives.
  Now support an additional parameter
      'bool constant_fallback = ...'
  to return "constant" or an empty word.

  The method signature changes slightly with a new optional bool
  parameter:

      //! Return \c "constant" instead of \c "" if the search failed
      const bool constant_fallback = true

ENH: code consolidation for findInstancePath

- relocate from Time to TimePaths and provide an additional static
  version that is reused in fileOperations

BUG: distributedTriSurfaceMesh:::findLocalInstance broken (#3135)

- was not checking the parent at all.

COMP: remove unused findInstancePath(const fileName&, ..) method