HPCC-10693 - Allow CSV,HEADING read in child query.

Normally a CSV with HEADING is marked global, meaning slaves
inter-communicate. However, if in child query, child reads all
parts and it is local.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5298 from jakesmith/hpcc-10693

HPCC-10693 - Allow CSV,HEADING read in child query.

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>