break beta div into two computations

[d-callan]

From microbiomeComputations created by asizemore: VEuPathDB/microbiomeComputations#21

When revisiting how the app works, break up the compute into two very clear distinct parts. Prep for in the future when we might have only the first part of the current beta div app running

[asizemore]

I'm working with beta div right now and am a little frustrated about only getting the pcoa outputs.
Did we have plans for how this would work? Some ideas in no particular order

1. Extra arg to the beta div function that says give me distances or pcoa output
2. Actually separate beta div into a beta div computation and a dimensionality reduction option
3. Return both pcoa and distance matrix in the compute result. Distance matrix could go in stats i s'pose, similar to the correlation output.

[d-callan]

those arent mutually exclusive. my suggestion, do all of the following:

1. add a distanceMatrix method
2. refactor the existing betaDiv method so it calls distanceMatrix
3. add an arg to betaDiv so that it can optionally, additionally return distance matrix data in the stats slot

[d-callan]

(how to organize it on the site, should we ever decide to let ppl plot the distance matrix directly, is less clear)