new wordRe class - a word that holds a regExp

  - a possible future replacement for keyType, but the immediate use is the
    wordReList for grepping through other lists.
  - note that the argList treatment of '(' ... ')' yields quoted strings,
    which we can use for building a wordReList

minor cleanup of regExp class

  - constructor from std::string, match std::string and
    operator=(std::string&)
    rely on automatic conversion to Foam::string
  - ditch partialMatch with sub-groups, it doesn't make much sense