TPCFastTransform: fix compilation on GPU with the new splines

[davidrohr]

@sgorbuno : This fixes the compilation of the new splines on the GPU. Things I had to change:

• local is a keyword and should not be used as variable name.
• Add GPUd() to all functions to be used on GPU.
• Fix some compiler warnings.
• While std::array works with OpenCL, it seems structured binding cannot work by design, since they rely on std::tuple_element, which is not supported. I thus had to change the code slightly to not use structured bindings.