Do not set strata if they appear to have been set earlier

`anova.cca` generated permutation matrix with `strata` in
`getPermuteMatrix` and passed that on to other functions (like
`permutest.cca`). This was broken in efd575f1 which passed
strata from `anova.cca` (unnecessarily), but we fix it in
`getPermuteMatrix` so that the solution is more general (and
`strata` are to be deprecated in the next releases in any case).

This fixes issue #71 that was generated by changes in #59