Implement ChainEvaluator

[AngelFP]

This PR implements a new ChainEvaluator that enables the workflow described in #87 . That is, it allows for evaluations consisting of several steps, where each step is carried out by a different TemplateEvaluator.

A use case for this are evaluations that require a series of simulations with different codes (e.g., FBPIC for generating a beam in an LPA, beam transport with ASTRA, and then beam interaction with a target in Geant4). Each simulation ( TemplateEvaluator) can request a different number or resources (number of processes and GPUs).

Here is an example illustrating how this evaluator would be used.

# Create one evaluator per simulation code
ev_fbpic = TemplateEvaluator(
    sim_template='template_simulation_fbpic.py',
    analysis_func=analyze_simulation_fbpic,
    n_gpus=2
)
ev_g4 = TemplateEvaluator(
    sim_template='template_simulation_geant4.txt',
    analysis_func=analyze_simulation_g4,
    executable='geant4',
    n_procs=40
)

# Group them in a chain evaluator in the order in which they should be executed.
ev = ChainEvaluator([ev_fbpic, ev_g4])

# Create exploration.
exp = Exploration(
    generator=gen,
    evaluator=ev,
    max_evals=100,
    sim_workers=4
)