BUG: error with empty distributed roots specification (fixes #2196)
- previously used the size of distributed roots to transmit if the case was running in distributed mode, but this behaves rather poorly with bad input. Specifically, the following questionable setup: distributed true; roots ( /*none*/ ); Now transmit the ParRunControl distributed() value instead, and also emit a gentle warning for the user: WARNING: running distributed but did not specify roots!