See merge request !509

This reverts merge request !524

This reverts merge request !524

Closes #5

- Remove IRegistry interface, and define it (for now) as an alias to RegistryEntry
- Remove reference counting from RegistryEntry, replace with std::unique_ptr
- Add move constructor/assignment to RegistryEntry

See merge request !524

See merge request !526

The syntax of
```
  ToolHandle<IMyTool> m_tool;
  bool m_useToolFlag;
...
  ATH_CHECK( m_tool.retrieve( DisableTool{ !m_useToolFlag } ));
```

Add another TaggedBool to  make it a little clearer what are people's
thoughts on `DisableTool / EnableTool` vs.
`DisableToolIf / EnableToolIf`:
```
   ATH_CHECK( m_tool.retrieve( EnableTool{ m_useToolFlag } ));
```

Add support for Gaudi::Range_ to DataObjectHandle

See merge request !571

add TimelineSvc-based implementation of 'last running schedule' detection (used in scheduler state dumps)

register (partial) timeline events as early as possible - right before the start of Algorithm::execute()

TaggedBool: use a more specific header guard name

See merge request !584

[[ deprecated ]] does not compile with using statements

See merge request !587

make uncompiled exported headers compilable

See merge request !590

Demoted a few AvalancheSchedulerSvc messages

See merge request !576

remove unused IIncidentListener inheritance from RndmEngine

See merge request !573

Revisit in-failure and in-stall console dumps of scheduler state, plus few other misc. things (mr !541)

*  remove double console dumps of the scheduler state occurring in a failure/stall
*  minor cleanup of associated scheduler's state-dumping logging messages
*  improve the coverage of logging protection by msgLevel(X)

And several other things:

*  re-implement PRG::accept() to walk visitors through all nodes
*  make StatusCode state checking explicit
*  remove unused and obsolete headers

required by an upgrade of autopep8