Extend event based epocher to handle multiple events

This allow the specification of multiple events in the StimulationBasedEpoching box. This is the best workaround I can find to avoid the bug in Streamed Matrix Multiplexer which does not preserve the order of chunks. See issue #52 final comments (the issue title is a little bit misleading, the classifier message is not spurious, it was appearing because of a Streamed Matrix Multiplexer).

This change allows the user to specify multiple events for the same StimulationBasedEpoching box. This has the advantage that the various epochs have by construction the same duration and offset parameters and the order of the various epochs is preserved. As such, this change removes the main need of Streamed Matrix Multiplexer (per its documentation). This also simplifies scenarios quite a bit.

Tested on fedora with the P300 speller (athena) which otherwise was experiencing various crashes because it triggered the chunk order check... I was first thinking that exiting OV was a little bit too much, but then figured out that we had a hidden bug because of this wrong order in the previous OV releases because it was not ensured that the classification probabilities were provided in the same order as the events...

There are two small obvious unrelated corrections....