Compute the normalization value for a field, given the accumulated state of term processing for this field (see
FieldInvertState).
Implementations should calculate a float value based on the field state and then return that value.
For backward compatibility this method by default calls LengthNorm(String, int) passing FieldInvertState.GetLength() as the second argument, and then multiplies this value by FieldInvertState.GetBoost().
WARNING: This API is new and experimental and may suddenly change.