ENH: provide getter/setter interface to standard communicators

- similar to UPstream::parRun(), the setter returns the previous value.

  The accessors are prefixed with 'comm':
  Eg, commGlobal(), commWarn(), commWorld(), commSelf().
  This distinguishes them from any existing variables (eg, worldComm)
  and arguably more similar to MPI_COMM_WORLD etc...

  If demand-driven communicators are added in the future, the function
  call syntax can help encapsulate that.

  Previously:

      const label oldWarnComm = UPstream::warnComm;
      const label oldWorldComm = UPstream::worldComm;
      UPstream::warnComm = myComm;
      UPstream::worldComm = myComm;
      ...

      UPstream::warnComm = oldWarnComm;
      UPstream::worldComm = oldWorldComm;

  Now:
      const label oldWarnComm = UPstream::commWarn(myComm);
      const label oldWorldComm = UPstream::commWorld(myComm);
      ...

      UPstream::commWarn(oldWarnComm);
      UPstream::commWorld(oldWorldComm);

STYLE: check (warnComm >= 0) instead of (warnComm != -1)