Feature/fraction digits

[lexjansen]

This PR makes the following changes to the schema (ODMv2 meeting notes:)

• Added fractionDigits as an optional item attribute
• Made type and label required attributes for items
• Allow null for optional attributes (length, fractionDigits)
• Add decimal and boolean to the item types.
• implemented itemGroupDataSeq by requiring that the first value for a record is a number:

"itemData": [ [1, "MyStudy", "001", "DM", 56], [2, "MyStudy", "002", "DM", 26], ... ]

This will also mean that the first item description needs to describe itemGroupDataSeq:

"items": [ { "OID": "ITEMGROUPDATASEQ", "name": "ITEMGROUPDATASEQ", "label": "Record Identifier", "type": "integer" },

See attached example. dd_example.txt