Preventing OOM by memory usage prediction

Over 70 GB of memory are used to build an aggregation Merkle sum tree with $2^{28}$ entries in summa-aggregation. 
This means that a Summa-aggregation process would be terminated without any error message on a machine with only 64 GB of memory.

We need to add some warning for memory usage when initiate `create_aggregation_mst` method.