deprecated `unifrac` function is calculating different results

The `unifrac` function (that is now deprecated but still works) is calculating in the latest version (2.0.13) different results from previous versions of the same function. Version 2.0.10 produced identical results as versions 1.0.x. This change can cause erroneous results for people that use code written before the version 2.0.13 (i.e. up to a week ago) and haven't had time to update it. As the underlying function is `bdiv_distmat` this may mean that there is a bug in the C code or R code of this function that were applied in 2.0.13. To avoid wrong results this function should be changed back to what it used to be or generate an error upon use