Zura Benashvili

Graph now will more accurately represent control flow in state machine.

Implement [Dgraph's Math on Value Variables](https://dgraph.io/docs/master/query-language/#math-on-value-variables). blocked by: #3

Core: implement @groupBy directive

2

Implement Dgraph's [@groupBy](https://dgraph.io/docs/query-language/#groupby) directive. blocked by: #4 since only aggregations can be run inside grouped edge.