change name of differentialAbundance method

[d-callan]

we have two differentialAbundance methods across our R ecosystem. theres one here, and one in MicrobiomeDB. this one is used by the site, and internally by the one in MicrobiomeDB. since the other is the one user-facing, im renaming this one. that should help clarify their relationship, but also make it easier for people looking through the docs to know what they need to look at to perform a diff abund analysis.

[asizemore]

Changes look good.

Question. This isn't our only example of having an internal version and a user-facing version (thinking about correlation here), and probably there will be more. Would you be happy with this naming pattern of "internal..." continuing?

Another question, are you concerned with anyone using microbiomeComputations straight, without MicrobiomeDB? Or is it too tied to MicrobiomeDB to really use by itself anymore (outside of our eda world)

[d-callan]

if a person wanted to use it directly, they could obviously, but i dont see a reason to cater to them. i think the docs should be clear as possible so people who go snooping can figure out whats going on, but thats about as far as i think we need to go. as for naming, i can be ok w it under the right circumstances. id like to avoid it as much as possible, but not to the point of bending ourselves or our docs into uncomfortable shapes. correlation i think is maybe a bit different, bc the api is consistent enough there that they can all reasonably use the same generic and so docs make more sense.