RF:  move .data -> eelbrain namespace (& RM plot.coreg)
FIX TEST:  test_examples, decrease number of permutations in MEG examples
DOC
RF plot:  lazy wx import
API:  retire Dataset.N
FIX make test:  run all tests
SETUP:  add extras_require
TEST anova
FIX _corr():  for one dimensional input
DOC anova:  NotImplementedError for empty cells