Lookup
Represents different behaviors of key lookup in series. For unordered series,
the only available option is Lookup.Exact which finds the exact key - methods
fail or return missing value if the key is not available in the index. For ordered
series Lookup.NearestGreater finds the first greater key (e.g. later date) with
a value. Lookup.NearestSmaller searches for the first smaller key.
