Make CUDA extensions optional

[ligerlac]

Hello everyone,
this PR makes the compilation of the CUDA extensions optional and allows to use the pure python implementation without installing cuda. I made the following changes:

• adapt setup.py so that building the extension difflogic_cuda is optional (still built by default)
• introduce the env variable DIFFLOGIC_BUILD_CUDA_EXT (set to false to disable extension) and update installation instructions accordingly
• wrap all imports of difflogic_cuda in try-except statements that throw an import warning if the CUDA extension was not installed
• In experiments/main.py, use the argument args.implementation to control what implementation to use (already existed, but was not actually used)