Use epochs.xml instead of epoch.xml

Both my MFF files and Ingrid's have epochs.xml, not epoch.xml, so fieldtrip didn't deal correctly with discontinuous data. 

So, I changed all the calls to orig.xml.epoch into orig.xml.epochs. I also added a "catch" in case the dataset contains "epoch.xml"

see http://bugzilla.fcdonders.nl/show_bug.cgi?id=2151