Reproducibility - Fix random seed on bsb

[drodarie]

In order to be able to guarantee reproducibility of reconstruction and/or specific bug, as a user, I would like to be able to set a random seed to bsb. All reconstruction steps are concerned.

[Helveg]

This will require a technical proposal to address RNG in the framework.

I propose:

• A new root dict rng which can contain RandomGeneratorNodes
• Each RandomGeneratorNode should provide an init_rng method that initializes their own random generator

We can create multiple subtypes of RandomGeneratorNodes like NumpyRandomGeneratorNode and NestRandomGeneratorNode that can initialize and pin down random number generation for these libraries we use, and provide a default scaffold one that users can use with self.scaffold.random and can configure from the rng node:

rng:
  scaffold:
    type: numpy
    generator: PCG64
    seed: 15
  nest:
    type: nest
    generator: Philox_64
    seed: 100

This would make self.scaffold.random a numpy PCG64 generator with seed 15, and would set nest.rng_type = 'Philox_64' and nest.rng_seed = 100. We should follow numpy's Generator interface. In the case that we're only setting RNG stuff for a dependency that type of node can just raise NotImplementedErrors