Spatial/Series refactor tracker

[Lun4m]

Per @intarga comment on #66.

• Unify SeriesCache and SpatialCache into a single data structure #70
• Change DataConnector trait
• Merge validate_spatial and validate_series
• Filter out flagged data from tests
• Tackle validate_fresh → moved to implement validate_fresh #77

[intarga]

Updates to this issue as things have changed.

Points 2 and 3 are resolved by #76

Point 4 is no longer valid, since we decided that filtering should happen inside the relevant tests themselves. This becomes an Olympian issue.

Point 5 I'm splitting out into a down-prioritised new issue (#77), since I realised validate_fresh is just a special case of validate (earlier referred to as validate_periodic). It's probably still worth doing, but I'm considering it as an optimisation rather than a structural blocker.

As a consequence I'm marking this whole issue as resolved by #76