add comparisons operator to the Period object and fixes issue 2781
move the Period tests in test_period.py and add assertRaises tests for comparisons
use a function factory for comparisons of Period objects
Merge remote-tracking branch 'Moisan/period_bug'

* Moisan/period_bug:
  use a function factory for comparisons of Period objects
  move the Period tests in test_period.py and add assertRaises tests for comparisons
  add comparisons operator to the Period object and fixes issue 2781
DOC: release notes