ENH: provision for delayed reading of compound tokens (#2953)

- the construction of compound tokens is now split into two stages:
    - default construct
    - read contents
  This permits a larger variety of handling.

- the new token::readCompoundToken(..) method allows for simpler
  more failsafe invocations.

- forward resize(), read() methods for compound tokens to support
  separate read and population.
  Top-level refCompoundToken() method for modify access.

ENH: split off a private readCompoundToken() method within ISstream

- this allows overloading and alternative tokenisation handling for
  derived classes

applications/test/charList/Test-charList.C

@@ -58,8 +58,8 @@ int main(int argc, char *argv[])
 
     #include "setRootCase.H"
 
-    Info<< "Known compound tokens: "
-        << token::compound::IstreamConstructorTablePtr_->sortedToc() << nl;
+    // Info<< "Known compound tokens: "
+    //     << token::compound::emptyConstructorTablePtr_->sortedToc() << nl;
 
     OStringStream ostr;