Choose middle eigenvector instead of minimal

This should probably be done only when MEG data is supplied. If EEG or combined
EEG and MEG data is supplied, the minimal egeinvector should be used, but this
needs to be tested.
Return absolute value of solution for optimal orientation
Changing name optimal to max-power
Remove pointless tests of orientation picking

Other tests should be developed.
Revert beamformer volume example

Reverting example in order not to make it too complicated with all source
orientation options. Instead a comment describing the other options is added.