WIP: Concat epochs from raws

@dengemann it should basically take a list of raws and a list of events, and construct an Epochs from it. It doesn't fully work yet. Feel free to scrap this if this isn't a good approach.

Basically we save our data for, say, 10 runs. I want to be able to concatenate them either at the raw stage, or the epochs stage, so I can analyze them all together. WDYT?