ENH MneExperiment:  `cov-rej` - customize `rej` method for `cov-file`
DOC:  eelbrain.experiment (WIP)
API load.fiff:  make evoked_ndvar() consistent with other sensor data functions
FIX MneExperiment:  `fwd-file` does not depend on `cov`
API MneExperiment:  load source estimates as SourceEstimates or NDVar; BK ttest
DOC MneExperiment
ENH MneExperiment:  register more make handlers
ENH data-obj:  add check_length()
FIX design:  random_factor with non-exact balancing