WIP ENH: Add event histogram example

I have some failing tests, not yet ready for review.
@mainakjas any reason you duplicated code here, as opposed to refactoring the code in `Epochs` to go in `_BaseEpochs` and then using that method directly, as I now do here? The duplicated code ended up causing a failed test, since I changed how the code worked in one place (`Epochs`), not knowing that the code had also been replicated here...