Fix: count with binning using multiple selections

[JovanVeljanoski]

Closes: https://github.com/vaexio/vaex/issues/2151

• [x] Implement a test
• [ ] test passes

[maartenbreddels]

Ok, had to think about this, but I'm not sure what to do here.

a single aggregation will only be done on a 'fixed grid', so for both selections the same min and max. At some point, we started respecting the selection for the min and max of the binby grid, now I think this was a mistake, because we painted ourselves in a corner. We cannot have two different limits for the two different selections.

[maartenbreddels]

@JovanVeljanoski we discussed this privately, do you remember what we decided?