HPCC-10147 Initial implementation of JOIN(..., GROUP(x,y))

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-10147 Add comments to examples to indicate the expected errors

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4959 from ghalliday/issue10147

HPCC-10147 Initial implementation of JOIN(..., GROUP(x,y))

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>