text: Title: Two warning tests fail
  Body: Running `pytest` on 1.3.13 I get the following two failed tests:
  
  ```
  _____________________________________________________________________________________________________________________ test_odd_num_levels_raises_warning ______________________________________________________________________________________________________________________
  
  setup_param_file_with_groups = '/tmp/tmphtdhaumg'
  
      def test_odd_num_levels_raises_warning(setup_param_file_with_groups):
          parameter_file = setup_param_file_with_groups
          problem = read_param_file(parameter_file)
          with warnings.catch_warnings(record=True) as w:
              # Cause all warnings to always be triggered.
              warnings.simplefilter("always")
              # Trigger a warning.
              sample(problem, 10, num_levels=3)
              # Verify some things
  >           assert len(w) == 1
  E           assert 2 == 1
  E             +2
  E             -1
  
  tests/sample/morris/test_morris.py:55: AssertionError
  _______________________________________________________________________________________________________________________ test_even_num_levels_no_warning _______________________________________________________________________________________________________________________
  
  setup_param_file_with_groups = '/tmp/tmpiur667kp'
  
      def test_even_num_levels_no_warning(setup_param_file_with_groups):
          parameter_file = setup_param_file_with_groups
          problem = read_param_file(parameter_file)
          with warnings.catch_warnings(record=True) as w:
              # Cause all warnings to always be triggered.
              warnings.simplefilter("always")
              # Trigger a warning.
              sample(problem, 10, num_levels=4)
              # Verify some things
  >           assert len(w) == 0
  E           assert 1 == 0
  E             +1
  E             -0
  
  tests/sample/morris/test_morris.py:70: AssertionError
  ```