Fix unsynced status of DownsamplingScheduler

[XianzheMa]

As we have multiple selector instances, the status of DownsamplerScheduler, i.e. which sampler is currently applied and which one should be used next, is out of sync. We should fix this.

Relevant PR: #496 (review)

[XianzheMa]

Actually, I want to completely remove the DownsamplingScheduler, as we also have never used it in experiments.

I think the design of DownsamplingScheduler, i.e. hard-code specifying "from trigger 3 to trigger 7, I want to use Downsampler X" by a user is hardly practical as the user cannot predict it in advance. We might want to have a more sophisticated scheduler based on data distribution, but this is out of scope for now.

What do you think @MaxiBoether ?