extra-data-validate: Condense similar problems?

[takluyver]

The output from extra-data-validate can be quite long, because a problem recording a run can affect multiple files & datasets in similar ways. @tmichela suggested on #188:

I wonder if we should try to summarize similar problems to avoid returning N times the same output with just a different source name, ex:

Index referring to data (11) outside dataset (0) for 82 sources, example:
  dataset: CONTROL/SPB_IRU_AGIPD1M1/CTRL/MC1/module2/dBoardTemperature2/timestamp
  file: /gpfs/exfel/exp/SPB/201901/p002316/raw/r0174/RAW-R0174-DA02-S00006.h5

and add a verbose argument.