Field part reference for compound field types

[Stiivi]

Allow use of parts of compound/indexable field types such as dates and arrays in operations. Example:

p.filter_by_value(FieldPart("event_date", "year"), 2013)

Advantages:

• less steps, no need to explicit extraction
• better readability

Disadvantages:

• more requirements for implementing backend operations
• operations might implement this selectively, depending on argument, which might cause inconsistencies

Requirements:

• Field.is_composed() - True for date, array, record
• DataObject.concrete_field(field_or_part)

Affected methods:

• prepare_aggregation_measures()
• prepare_key()
• many operations

Recommendation: have this in OperationContext when argument annotations or when operation prototype metadata are implemented.