HPCC-12187 Move classes for gathering selections of statistics

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6520 from ghalliday/issue12187

HPCC-12187 Move classes for gathering selections of statistics

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>