Ensure epochs being concatenated have compatible event ids

[larsoner]

I hit a sneaky little bug where I had a variant of

epochs1.event_id == {"a": 1, "b": 2}
epochs2.event_id == {"c": 1}

and MNE-Python silently allowed me to concatenate these using concatenate_epochs. It led to really weird scenarios where __getitem__ access (e.g., to epochs_concat["b"]) would result in a mixture of events. This adds a check both at the concatenate_epochs stage (public API) and at the __getitem__ stage (in case users have messed with event_id... which they aren't really supposed to do but might as a workaround) to do some sanity checks.

[larsoner]

This is a pretty straightforward bugfix so I'll self-merge this one!