IdGenerator prevents parallel mutation of genomes

[SilvanCodes]

As of now genomes can only be mutated sequentially as the mutate method of the Mutation struct requires exclusive mutable access to an IdGenerator. The IdGenerator is a shared resource, i.e. it has to be the same for all mutation operations on all genomes as it is responsible for tracking and managing node identities in order to determine genome similarity or 'distance' between genomes. This is used to approximate genome compatibility for crossover which assumes matching structure via node identities.

If genomes are used without crossover it could be possible to avoid having a shared IdGenerator.

If crossover is desired, the IdIter could be rewritten to utilise an Arc + RwLock on the IdGenerator and only acquire the write-lock when a new Id needs to be generated and written to the cache.