Query rework

Rework Query object to more easily and explicitly handle parenthetical
grouping and AND/OR-ing of terms. Update Forge methods to match. Update
tests to match new Query and Forge structure.

Rearrange Forge methods to display new structure: Core methods use
Query directly, Helper methods use the Core methods to make certain
common searches faster to build, and Premade Searches leverage the Core
and Helper methods to build and execute a complete search.

Also minor aggregate bugfix, which now explicitly only returns
mdf.resource_type:record entries, and not dataset entries.