Allow continuation of previous exploration

[AngelFP]

A workflow we should support is the ability to continue a previous exploration. This can be done once Libensemble/libensemble#1041 is merged.

Apart from resuming an old exploration, this would allow for workflows such as

exp = Exploration(
    generator=gen,
    evaluator=ev,
    max_evals=1000,
)

# Run the first 100 evaluations
exp.run(n_evals=100)

# Do something (e.g., look at surrogate model, change hyperparameters, ...)

# Run the next 100 evaluations
exp.run(n_evals=100)

# etc.