fix event storage for concurrent execution

[m-fila]

BEGINRELEASENOTES

• Added Event and Dataflow structs
• Fixed event storage for concurrent execution

ENDRELEASENOTES

The pipeline running function introduced in #29 was reusing the parsed graph (graph topology and description) instead of creating a new instance for each event. It went unnoticed that the schedule_graph methods mutates the graph by using it for storing algorithms` futures.

As a fix two new structs are introduces:

• DataFlowGraph to held immutable information about graph topology and algorithms, and possibly do some graph topology caching
• Event holding mutable event specific storage and access to immutable DataFlowGraph
• schedule_graph is renamed to schedule_graph! to indicate it mutates Event