StatusCode Explicit Definitions, master branch (2019.01.04.)

The explicit definitions are needed to be able to use (for instance) constant references to these constants in user code in C++14 mode. In C++17 mode the explicit declaration is not needed anymore. But it also doesn't hurt the compilation. (I checked. 😛)

Note that this issue was discovered in atlas/athena!20786 (merged).

Tagging @krumnack, @fwinkl, @ssnyder and @leggett.

P.S. I was a bit unsure how to set the noinline attribute on the test function. Since I saw many examples in the code using this formalism, I went with this one. But if you have a better suggestion, I'm of course open to something else as well.