* introduce CounterHolder, which keeps track of the
  available counters, and provides declareCounter,
  findCounter, forEachCounter, nCounters and clearCounters

Closes #54

Also applied to profiles.
This will of course slow down slightly the current histograms. On the other hand, they should be replaced at some stage with the new ROOT7 ones, so this is kind of temporary

See merge request !820

See merge request !812

they help with debugging

if a .new file already exists, use an incremental new name