ENH testnd:  compute clusters without permutation
ENH cluster tests: implement advanced cluster properties as Dimension methods

- FIX tmin criterion with non-adjacent dimensions
ENH  load.besa.dat_set():  allow * in path
ENH Factor:  add `.endswith()` method
ENH _ClusterDist.__repr__()
ENH combine():  Datasets inherit info (intersection)
FIX load.tsv():  empty string cells