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).