Makes DataObject refcounting thread-safe.

(Not used in the ATLAS build, just for fcc.)

Missing #include <algorithm> for std::count.

Missing #include.

The TString header adds the overload:
    bool operator==(const char *s1, const std::string_view &s2);

This is fine as far as it goes.  But c++23 also adds a string_view ctor
from a Range, which the implementation qualifies with a requires
clause to enforce that the type models a range.  The upshot is taht
if we have a Property<int> p and try to do, say, p != 0, then
this will try to instantiate Property<int>::begin().  This will
of course fail.  So we need to ensure that the begin/end methods
of Property are enabled only if the wrapped type is also a range.

- Implicit assignment op in the presence of an explicit
  copy ctor is deprecated.  And also the other way round.
- struct/class mismatch.
- unsigned / signed comparison.

See merge request gaudi/Gaudi!1717

See merge request gaudi/Gaudi!1716

See merge request !1714

Note that this is not a proper fix but rather a workaround.
It seems that this version of local_valgrind is outdated and not able to properly detect modern ARM platform.
Besides one could wonder why we have to replicate a header from valgrind here.

Closes #344

See merge request gaudi/Gaudi!1715

See merge request gaudi/Gaudi!1705

Modify ServiceHandle to fall back on dynamic_cast if the template
argument is not an abstract interface. This is similar to the way
ToolHandle works for concrete tools.

This also adds the new header Gaudi/Concepts.h to host C++20 concepts of
common utility, starting from Gaudi::IsInterface<T>.

Closes #357

See merge request gaudi/Gaudi!1708

Closes #354

See merge request gaudi/Gaudi!1713

See merge request gaudi/Gaudi!1711

See merge request gaudi/Gaudi!1707

See merge request gaudi/Gaudi!1704

See merge request gaudi/Gaudi!1703