prep design for parser class
separate AggregatorSet from Interval
IntervalBuilder takes a parser_columns argument instead of a parser class
no need for GroupAndCount class, merge it with Count, let aggregators register themselves
load and lookup aggregator classes based on calls they make to register_aggregator. Custom aggregators now behave like evry other aggregator
auto register aggregators when they are loaded (through the inherited hook)
all classes of aggregators can take options
extract LineParser class methods into a module
rename LineParser to ParsedLine
cleanup examples, design, readme. Allow IntervalBuilder to take a Logfile or a LogfileSet as an argument
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