GPU Support

[acalejos]

Need to implement GPU support. This includes:

• Add CUDA flag in the build
• Decide on an approach to precompilation with CUDA
• Implement CUDA NIFs in C and Elixir side
• Add Elixir-Side support for training and predicting with CUDA
• Add in CUDA params

Some of the relevant API resources:

• https://xgboost.readthedocs.io/en/latest/c.html#xgboost-c-package
• https://xgboost.readthedocs.io/en/latest/build.html#building-with-gpu-support

[acalejos]

Removing this from 0.3.0 milestone seeing as that release already has quite substantial additions and we would like to push those out