SummarizerBase performs work shared by all sumarizers, most everything except for handling of unique keys.
The base class handles creation, allocates storage for Operators and SharedFieldValues, and similar. Derived classes deal primarily with unique keys and the associated Calculators and UniqueKeyValuesLists.
Called to process the header line of each file. Returns true if it was the first header line processed (used when reading multiple files).
Sets the Operators used by the Summarizer. Called after construction.