- do not rely on LCG clang-format installation
- do not require cmake configuration

Closes #57

* 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