Better Docs for individual_/genome_ operators

[escott-mitre]

We have an unfortunate and confusing pattern of having operators (which apply to iterators) named my_operator and helpers that they are build from (which apply to a single genome or individual) named things like individaul_my_operator or genome_my_operator.

This affects users particular when they want to work with segmented genomes, since the segmented.ops.apply_mutation() expects one of these strange little helpers to be passed in (and not a regular operator).

• At a minimum, improve the docs on these functions to explain the difference better.
• Better yet, find a way to refactor things so this confusing distinction is no longer necessary (or is made obvious by better names)