ENH testnd:  don’t precompute clusters

- ClusterDist has .clusters() method
- TestResult have .clusters as LazyProperty
ENH NDVar:  make .__getitem__() use .sub()
ENH NDVar:  Allow boolean NDVar as argument in summary methods
ENH Var, NDVar:  support __array_interface__
ENH MneExperiment:  add cluster sum box plot and pairwise tests
ENH ClusterDist:  store .cluster_map as NDVar
OPT testnd:  clustering workers

- os.nice(20)
- for single distributions use cpu_count()
- for ANOVA use ceil(cpu_count() / n_effects)
CLN NDVar:  .get_dim() and ._align()
ENH UTS:  make .dimindex() and .intersect() exact

use tmin, step and samples instead of times
ENH NDVar._align():  allow for dimensions that are unequal but intersect