Add CUDA-wrappers submodule to replace util/cu.* and util/nvrtc.*

Bram Veenboerrequested to merge
cuda-wrapper into master

The https://github.com/nlesc-recruit/CUDA-wrappers repository is added as external/cuda-wrappers git module. The Makefile is updated to run cmake and make for this module and to use the resulting libcu.so for the functionality previously provided by util/cu.* and util/nvrtc.*.

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