Style string methods

• Consolidate some string methods, improve consistency of const-access, use some C++11 constructs for more clarity.

• static versions of fileName clean(), path(), name(). The general idea is that there are number of methods related to fileName semantics but that we may wish to use on any existing string type (eg, std::string or Foam::string without transforming to fileName first). Among other things, this lets us cleanup a string like a fileName and split it, but without requiring that the individual elements are a word. We may wish to have a keyType instead (containing invalid word characters).

merged