Calculating gunifrac on the fly in ord_explore gives no indication that it is working

[david-barnett]

With many hundreds of ASVs and hundreds of samples, interactively selecting the unifrac distance options and clicking build appears to hang indefinitely because there is no indication to the user that calculation has begun.

The user should ideally be shown a waiting spinner of some kind and warned, if there are over some arbitrary number of OTU table cells, that the computation will take a lot of time and should better be done in advance and perhaps saved to disk.