Make CUDA optional for at least the main lib

[rpavlik]

• Make CUDA optional in the build system.
• Make lm_optimizer.h compile without cuda if desired.
• Slight cuda-related CMake cleanup

I wasn't able to easily make the app build without cuda, unfortunately. Are there any plans to support that? I'll push my WIP.

[puzzlepaint]

Thank you very much for this and the WIP PR. It seems to me that this one should be merged only once the other one is also ready, since libvis on its own is probably not really useful, and thus the CMake error message about the missing CUDA SDK in the top-level CMakeLists.txt should probably be kept as long as the application does not compile without CUDA.

I wasn't able to easily make the app build without cuda, unfortunately. Are there any plans to support that?

In general, this repo was created to open-source the code for a specific project while I did my PhD. In the meantime, I finished the PhD and currently do not have any plans for active development of the project. However, I try to do basic maintenance (handling issues and PRs) in my spare time and will gladly merge a PR for making CUDA an optional dependency.