Terminate support for Cobalt workload manager

[al-rigazzi]

Description

We are discontinuing the support of Cobalt as a workload manager. This means that cobalt will not be a valid value for the launcher argument in classes such as Experiment and Orchestrator.

Justification

Given that the Theta supercomputer was retired and Theta was the only system we are aware of which was using Cobalt as a workload manager, we have made the decision to discontinue its support. This is mainly due to the impossibility of assessing whether the support is still working, as there is no system on which we can test it.

Implementation Strategy

PR #448 removes support for Cobalt in every SmartSim class and consequently eliminates mentions of it in documentation. If any user is still using Cobalt on a system we are not aware of, we kindly ask to inform us. We will wait for two weeks - i.e. until January 22nd - before merging #448.