See GAUDI-1312

Fix a bug in data/condition object state detection in temporal and topological tracer's (3T) graph assembling

See merge request !543

See merge request !533

Adds a method to `IScheduler` to attach `EventViews` to the
`PrecedenceRulesGraph`, so that parts of the graph can be processed
multiple times per event (or skipped entirely).

No measurable performance impact for non-view workloads.

See:
- GAUDI-1306
- https://indico.cern.ch/event/632442/contributions/2718811/attachments/1526541/2387104/EventViews.pdf

- as `std::map` never invalidates pointers to values, there is no
  need to use `std::unique_ptr<Mutex_t>` instead of plain `Mutex_t`
  as value

See merge request gaudi/Gaudi!517

See merge request !512

Modernize constructors and destructors

See merge request !515

See merge request !456

See merge request !485

See merge request !490

See merge request !513

- remove destructors qualified with 'override = default'
- prefer inheriting constructors

... plus some formatting tweaks,

See merge request !519

See merge request !521

See merge request !522

See merge request !528

See merge request !516

See merge request !527

- remove redundant destructor
- prefer inherited constructor
- avoid std::shared_ptr, emplace actions into the queue
- prefer lambda over std::bind

- inherit from Algorithm instead of GaudiAlgorithm
- remove redundant initielize, destructor

Set the algorithm execution state/status (maintained by the
AlgExecStateSvc) in Algorithm::sysExecute. Clients calling sysExecute no
longer need to worry about setting this themselves (just like it used to
be in pre-GaudiHive times).

Type punning was implemented using a `union`, which relies on
writing to one member of the union, and then reading from another.
Strictly speaking this is 'undefined behaviour' in C++, as only
the 'active' member of a union should be used. However, this
works as a gcc extension, as it is valid C11 (note: no ++!).
This patch changes this to use `std::memcpy`, which casts (implicitly)
to `unsigned char*`, and casting any pointer to/from `unsigned char*`
is well-defined in C++.

Note that in practice, gcc and clang generate _identical_ code -
see eg. https://godbolt.org/g/3kwRbo - so from that point
of view this patch 'does nothing' but it is more standards compliant ;-)