Adding two new attributes for specifying basket size maximum and minimum.

This MR adds two attributes to allow customization of basket size: m_maxBufferSize restricts the amount of memory for a single buffer after the first flush (i.e. after ROOT optimizeBasket() was called) m_minBufferEntries increases the amount of memory for baskets to be able to hold the first n entries, should be called befor the first flush (i.e. n < auto_flush).