implement important activation layer types

[MichaelHirn]

The following activation layers have a high priority:

• Sigmoid
• ReLU

Other activation layers that are interesting for implementation:

• TanH

[hobofan]

With #49 the ReLU layer has been added.

[MichaelHirn]

ReLU, Sigmoid and TanH are now supported for CPU/Rust and GPU/CUDA via Collenchyma-NN. {GPU, CPU}/OpenCL should be tracked at the NN Plugin directly.

I believe this issue can be closed therefore. If necessary, I am happy to reopen it.