Add ordering to properties where required

[DanSmith]

This work is based on the requirements in #23

Ordering was added to:

• Item Groups
• Variables in records
• Group/Study/Datacollection structures
• Survey Instruments

[DanSmith]

I've reviewed the items listed in #49 for cardinality checks.

Several of the items on the list were actually 0..1 in the schema so no changes were needed. Items that had a mismatch in cardinality between the 3.3 schema and the model were updated in https://github.com/ddialliance/ddimodel/pull/48/commits/d5212b88a0dec8dcd47eb21b95196b913176bb88