Add support for EventContext as input to Gaudi::Functional algorithms

See merge request gaudi/Gaudi!671

See merge request gaudi/Gaudi!696

See merge request gaudi/Gaudi!695

Fixes LHCBPS-1792

it avoids spurious test failures on busy Gitlab-CI runners

CMake 3.11 looks for ninja in CMAKE_PREFIX_PATH and the SFT builds of it
requires libstdc++ more recent than the system version.

Gitlab CI authored May 25, 2018 and Andrea Valassi committed May 25, 2018

patch generated by https://gitlab.cern.ch/valassi/Gaudi/-/jobs/1471225

- prefer CommonMessaging when available
- add generic visit to downcast INTupleItem to concrete type and
  invoke the provided callable, and use that to dispatch lambdas
  instead of explicit switch statements with almost identical function
  calls
- use boost::string_ref to avoid copying strings more than strictly
  required
- prefer direct member initialization

- 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 gaudi/Gaudi!667

Closes #28

- one can now specify `const EventContext&` as the first (input)
  argument to a Gaudi::Functional algorithm
- the implementations will skip the `EventContext` when generating
  the input handles for the input, and when calling `operator()`,
  it will prefix the arguments with the result of
  `Gaudi::Hive::currentContext()`.

- use `apply` and `detected`
- prefer tag dispatch over template specialization
- fix InnerType <-> Arithmetic swap
- prefer boost::algortihm::icontains
- remove redundant constructor calls
- remove redundant copy c'tor and assignment declaration