FIX: Brainvision Reader

These are some modifications I made to get the Brainvision reader to work for me. The individual commits keep different issues separate, so if there are too many changes here I can send PR for the separate commits :) .

I made one modification that would break backwards compatibility when assigning coordinates to electrodes by removing the constraint that the first 3 positions have to be the fiducials and the next 5 positions are ignored (this seems to be specific to the KIT setup and might not be useful in other systems). https://github.com/christianmbrodbeck/mne-python/commit/ce99321bad1e92f893bbd2bec20b90ce114e7969#diff-1f7de16acaf2b7e832d4fb6893b52a1aL503