Automate permutation strategy

[kylebittinger]

For an experiment with repeated measures, the adonisplus() function should be able to determine a sensible permutation strategy from the data:

• Character vectors or factors that are constant within each subject should be shuffled between subjects.
• Character vectors or factors that vary within subject should be shuffled within subjects.
• Numerical vectors should have a free permutation.

With this in mind, we might change adisonisplus() to select the most obvious permutation strategy, while allowing the user to override if it's needed. To implement this, we would:

• Set the default value of rep_meas_var to NULL, so the function knows to guess the permutation strategy when the user supplies a value here.
• When rep_meas_var is not NULL, set up the permutions as above.
• When shuffle is not NULL, override the variables that are specified.
• Add an attribute to the output object providing info on the permutations.
• Add info on permutations when printing the object.