Setting CMAKE_CXX_STANDARD (for STIR, ITK etc)

[KrisThielemans]

Every package has different ways to set the C++ standard, which is problematic as libraries and .h files aren't necessarily compatible, leading to crashes or linking problems, see e.g. #161. It seems therefore safer that if we compile stuf ourselves, to try and convince the package to use a particular C++ version.

At present, we set CMAKE_CXX_STANDARD to 11 for STIR. This hasn't create problems yet, but I do get warnings with ITK (that ITK was compiled with a more recent C++ standard).

There's a few possibilities:

1. relying on the packages and their default choices for the particular compiler we're using (wouldn't currently for STIR)
2. attempting to enforce a C++ standard

Seems to me that we need to find out what C++ version is used for Gadgetron and build boost with the same version (as in #161), but also build ITK, STIR and whatever with the same version. I hope that for all CMake packages, this just means specifying CMAKE_CXX_STANDARD (I haven't check the ITK code).

[KrisThielemans]

This is important but I don't think urgent, and wouldn't break backwards compatibility. I';; therefore assign to v2.1, but please comment if you disagree.