Allow modifying extraction and collection

[wheresrhys]

To enable creating variants of a query before fetching, which in turn would enable full flexibility in creating aggregators which accept a single query, split it into a number of related ones, and then aggregate the result

e.g. @cohort() would concat return two queries, one filtered to exclude anon, one to include

required methods

• freeze, (which shoudl be called internally by print when fetching?) - indicates that any methods will henceforth generate a clone
• unsetExtraction
• setEvent