HPCC-10068 Implement syntax for JOIN(,SMART)

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPPC-10068 Various changes following review and new test case

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4888 from ghalliday/issue10068

HPCC-10068 Implement syntax for JOIN(,SMART)

Reviewed-By: Jake Smith <jake.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>