EHN: nodes bad_channels and bad_epochs

* Documentation for bad_channels has been updated

* New docs for bad_epochs node

* Config options Statistic1 and Statistic2 for node bad_epochs have been renamed
to the more meaningful ChannelStat and EpochStat.

* Config option Percentile and MADs have been removed from all criteria (for
rejection of channels or epochs). These two options were redundant and can be
now implemented using options Min/Max