Resolve "Rework "Rate Stats""
Closes #203 (closed)
Implements the databuffer to buffer values for better rate approximation: Enables getting rates over a longer period of time.
Introduced CounterAttr to replace Rate: Encapsulates all values (counter, different rates) for on counter. Migrated CollectedData objects to CounterAttr. Changed metric_funcs accordingly to get rates like before. New rates (approximated over a longer time are not yet used).
Communicator Rates will be migrated with Rework communicator stats (#205 - closed)
Additional Review: @usnlf_albrecht (metric_func)
Edited by Christian Dreher