Add support for replica execution in FabSim

[djgroen]

In discussions between me and @arabnejad we begin to realize that the current ensemble mechanism is not so efficient for executing the same simulation multiple times. This is because the user will need to prepare a large number of (near-) identical configurations, with one directory per run.

In addition, we see a clear case for running ensembles, where each sample of an ensemble is run multiple times to account for stochastic nature of the underlying code.

We therefore decided to implement replica execution both separately, and as part of the ensemble execution functionality in FabSim.

Tasks are as follows:

• Prototype fabsim <machine> dummy with replicas. (@arabnejad )
• Prototype fabsim <machine> dummy_ensemble with replicas. (@arabnejad )
• Port both implementations to FabFlee.
• Write documentation.
• Write test cases.

[djgroen]

@arabnejad I thought about this more, and I think it may be better to implement this as a flag than as a separate function name.

So instead of proposing:
fabsim localhost dummy_replicated:dummy_test,replicas=5
we could go for:
fabsim localhost dummy:dummy_test,replicas=5

Similarly, for combined ensembles with replicas, we could go for:
fabsim localhost dummy_ensemble:dummy_test,replicas=5

What do you think?

[djgroen]

This works for FabFlee. I also

• refactored the variability functions (deprecated them).
• wrote documentation for this in three places.