Add Factories::Graph.only_provenance stub factory
Graph#from_CSV is a instance method and reads 2 CSV's correctly

* also the string concat of 2 CSV exports is now tested as a valid
  logical concat of two graphs
* graph CSV dumps can now be manipulated by straight
  'cat dump_1.csv dump_2.csv > dump_1+2.csv' on the command line
  (as long as the time_stamps are non-overlapping).