Mbio: consider a spline fitting viz for diff abund

[d-callan]

this is a placeholder ticket to keep track of thoughts as we discuss priority and min reqs, etc.

the idea is to introduce a spline fitting viz to the differential abundance app. i have questions here about whether this might be better as a separate app. this isnt something im very familiar w yet, though, so maybe things will become clearer as i read.

[d-callan]

from dan: