guess consolidation function from whisper aggregation method or graphite aggregation functions

When displaying a metric defined in whisper as being aggregated by max, one expects the drawing consolidation function to be max too, whereas the default one is avg excepted if the graphite target explicitely specify consolidateBy(...,'max').

When displaying a bunch of metrics aggregated with e.g. maxSeries(), one expects the drawing consolidation function to be max too, whereas the default one is avg excepted if the graphite target explicitely specify consolidateBy(...,'max').

These commits guess consolidation function from whisper and set/override it when maxSeries() or minSeries() is used in graphite target.