Include support for opportunistic resources with dynamic availability

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It is currently not possible to simulate opportunistic resources, e.g. NEMO cluster, where the availability of resources is time-dependent. However, with the ability of WRENCH to simulate pilot jobs running on a cluster, including opportunistic resources into the simulation shouldn't be beyond reach.

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For the beginning, however, it should be sufficient to approximate them as static resources, when simulating workflows on small time-scales.