Modification at generate_transaction_graph.py to generate transaction graphs between a time interval

I'm suggesting a modification at `generate_transaction_graph.py`, as the original version got core dumped in several tests I have done. Therefore, I decided to analyse and to debug the script code and I found out that as bigger as the graph was getting, the OS(Linux) threw a trap and I got `core dumped` as a signal the graph allocation had overpassed the memory heap area as the script has built the graph since the genesis block. 

I picture that another benefit from this modification could be the flexibility that one could have once she can be interested in a specific time interval to make some analysis on a generated transaction graph.