Update naming for elements relating to potato set

[hfrick]

... once we are sure what we want to call things.

I've been missing a way to differentiate the analysis set of the split object going into inner_split() and the analysis set coming out of it so I've been referring to them as outer analysis set and inner analysis set, respectively. The inner assessment set in that "speak" is the potato set.

This relates to #490 and the labels but also the documentation as in #489 (comment)

[hfrick]

We'll go with "calibration set" since that is currently all we use it for and we don't necessarily anticipate using it for anything else.