ENH: handle unordered events
ENH: use warning instead
ENH: improve tests
COSMITS
Merge pull request #796 from dengemann/monotonic_events

ENH: handle unordered events + prevent non-chronological event schemes in Epochs
FIX: diff on empty / single events
WIP ENH 5D TF Beamformer example
FIX Checking array inequality should use .any()
ENH Add plotting to t-f beamformer example
Comment out overly restrictive frequency check

With this code uncommented the plot_tf_beamformer examples doesn't work,
because numbers of samples are sometimes slightly different for noise and data
time windows. Is this check really necessary? Maybe a different one should be
put in place or a warning issued instead of raising and error?
Change default number of tapers to 3
Several changes to WIP TF Beamformer example
Separate out tf_dics from example into a function
ENH Return stcs for frequencies instead of for time windows

This allows us to return stcs that have multiple time points instead of
multiple frequencies.
Slightly more forgiving tmin and tmax checks

i.e. now if tmax is specified very close to last time point (i.e. larger than
last time point, but smaller than where the next point would be expected), no
error will be raised.
ENH Time windows overlapping and different across frequencies
Several improvements to tf_dics

* Move noise_csd calculation out of loop across time windows to avoid
unnecessary computation
Using equal length time windows in example
Small code cleanup, remove control parameter
Rename _tf_beamformer to _tf_dics