Compiling with master ITK/RTK and C++14

[tbaudier]

With this commit in ITK (InsightSoftwareConsortium/ITK#2563) the minimal version of C++ is 14 instead of 11.

We have different possibilities:

1. Do not use Gate with master version of ITK/RTK, prefer release tag.
2. Force to use C++14 if the flags GATE_USE_RTK or GATE_USE_ITK are set like for torch (https://github.com/OpenGATE/Gate/blob/develop/CMakeLists.txt#L237).

What do you prefer?

[m-dupont]

I prefer the first one, and i would say, i propose to use the minimal needed release version of ITK/RTK. It make this more easy to compile it on old exploitation system. For example on centos7 or ubuntu 14, there are old but still widely used, specially in hpc or in my lab.

Use a specific version make also more simple the reproduction of a issue encounters by GATE users. Users just have to specify itk/rtk version used.

[SimonRit]

Thanks Thomas for #445. Just to be clear, we'll still have to fix the CI for the next ITK release. I'm not sure if the C++14 patch will be included in v5.2.1 (in a few weeks) or if that will wait v5.3 (in a few months / year). Doing what is done with Torch is an option although I think it would be preferable to let the user chose a compatible C++ version (14 or above).

[tbaudier]

Thank you for your answers, I fix the version of ITK to debug the CI and I let the user chose the versions of ITK and C++ he wants.