Use permute API in diversity accumulation models

This PR answers issue #88 and implements standard permute API for diversity accumulation methods that have `permutations` argument. I assume that these functions will rarely need anything but simple random permutation, but now their interface is consistent with other functions having `permutations` argument.

The change concerns functions `specaccum`, `poolaccum`, `estaccumR`, `renyiaccum`  and `tsallisaccum`.

In addition `estaccumR` function can now use parallel processing. Other functions are ready for plugging in parallel processing, but they are not particularly slow currently.