FIX: Fix multitaper computation

@dengemann thanks for the tip about `nitime`. This should work for both old and new `nitime` versions. @mluessi I think this was some of your code, you okay with the naming?

Ready for review and merge, hopefully quickly so that `master` can be fixed. Europeans/east-coasters, feel free to take over if it expedites the merge (e.g., while I'm asleep).