Move informer superstages to their own module

[aimalz]

This is mostly a heads up about the one aspect of LSSTDESC/rail#37 that touches on this repository, i.e. base code. I'm going to make a new module for the various Informer stage base classes because they're distinguished by taking some forms of data as input and producing a model file as output and are thus shared across algos; an algo with a stage for more than one of estimator/summarizer/classifier will share a single informer stage, yet its superclass will be imported from from just one of the estimator/summarizer/classifier modules. Under this change, new developers would just have one place to look to find the right informer superclass for their algo.

EDIT: Relatedly, src/rail/core/data.py should be renamed src/rail/core/handletypes.py or something like that so it's descriptive.