Avoid using multiprocessing by default

[mj-will]

bilby sets npool=1 by default, this means that if the user doesn't override the value, then nessai is passed n_pool=1 by default. This is inefficient and can break some analyses, e.g. if using a GPU. We should consider handling the case where n_pool (or equivalent) is not specified and npool=1 to avoid this.