Select starting nodes class (for multistart decoding)

[fedebotu]

At the moment, we have a select_start_nodes (here) that is not very modular (i.e. for each environment, you have to manually make an "if---else"). Also, it is not clear at all for newcomers. We should make this as a class such that a user can create their own and pass it.
In the same class, we should also include a method to get the number of starts for the environment (e.g. for PDP, we have half of the nodes as possible starting points)

Related: See #108

[LTluttmann]

what do you think of letting the environment classes define the select_start_nodes and get_num_starts functions?

[fedebotu]

I agree with you, this would make it more modular! Thoughts @cbhua ?

PS: maybe we don't need to enforce it for all environments, but add it only to envs that may require this feature (i.e. Implement documentation in RL4COEnvBase and raise error if the env did not implement it)