FIX MneExperiment.plot_annot()
OPT lm_fitter:  preallocate values array
OPT/STY lm_fitter:  precompute effects and dfs
STY LMFitter:  rename lm_fitter -> LMFitter; X-->x
OPT LMFitter:  pre-allocate result when splitting
API LMFitter:  .map() returns list of F-maps
API glm:  capitalization  lm -> LM, anova -> ANOVA
TEST LMFitter
OPT LMFitter:  pre-allocate everything
ENH hopkins_ems:  raise error if all effects are fixed