HPCC-8312 Rationalise the use of property and attribute

Attribute refers to an option explicitly added by the user

Property refers to something that can be derived from an
expression including explicit attributes.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4501 from ghalliday/issue8312

HPCC-8312 Rationalise the use of property and attribute

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>