- prefer boost::string_ref instead of const std::string& in interfaces
- add transparant comparison when using std::map, so that a string_ref
  can be used to search in a map which has keys of type std::string
- move standalone functions only visible in a single compilation unit
  into an anonymous namespace so that these functions are not visible
  to the linker

See merge request !667

Closes #28

Old definition was using the class keyword, the new one was initially using struct and so any forward declaration created a clang warning.
We are now back to class.

add support for multiple CF parents to topological execution flow tracer, and verbose/error messages cleanup in PRG

* master:
  Add test and fix of version propagation in KeyedContainer move (#27)

This avoids problems when the user has GIT_DIR and/or GIT_WORK_TREE set
in the parent shell.

Closes #27

To enable better migration of Algorithms to MT, detect if derived
Algorithm classes implement begin/endRun methods, and print a
warning message if so.

No checks should be necessary for MT code, only serial, as the
MT EventLoopMgr doesn't call Algorithm::sysBegin/EndRun.

This is a temporary check until all Algorithms have been migrated,
and we can remove the methods entirely.

* master:
  Fix changes list for formatting check

The new counters have a few new features compared to existing StatEntity :
  - they are templated by the counted type
  - they come with different flavors counting only what you need (e.g. simple count, no stat)
  - more flavour can easily be created at will thanks to the AccumulatorSet object
  - they come with 2 implementations : thread safe or not
  - the thread safe ones use atomics and should be as efficient as possible
  - a Buffer object allows to easily buffer updates to a counter made in a tight loop, without paying the atomic price each time

A Small unit test is available in the test suite and example code is provided in the comments of Counters.h where they are implemented.
Existing code has been adapted, in particulat the ChronoSvc.