Cosmological time integration

Merged Matthieu Schaller requested to merge cosmology into master

This is an absolute monster merge request.

It adds support for cosmological time integration. It has barely been tested for accuracy thus far. But I'd like to merge it in so we can move on with the accuracy tests. In principle, nothing is broken in the normal mode of operations and we should be able to run anything we currently do without any problems.

If this is confirmed and merged in, then I will update the examples we have to actually use the new module.

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