Implement seed management

[denniskb]

• All components that require it must be given access to deterministic, high-quality, sufficiently large seeds.
• It should be possible to chain seeds, to generate deterministic follow-up seeds from a master seed.
• It should be possible to cheaply generate streams from a seed.
• Provide seed as mandated parameter to all essential functions (ctor, update(), deliver()).
• Benchmark, esp. the perf. of static thread_local rngs. It might be cheaper to pass an rng directly into the afore-mentioned methods. (Only one has to be created per thread, not per neuron!). Still, prefer more flexible seed approach. Yep, much simpler! no need to worry about perf. Do benchmark std::seed_seq though! This will inform the design, whether a cheap chain-method is still required.

[denniskb]

std::seed_seq::generate() takes some ~65ns, so no concern for performance at all. Just implement a high-quality rng to cover all cases and pass a reference to that to user functions. (Could even turn into a struct of useful metadata (one mandated by snn, one, optional, user-specified).