Add a label for each reflection

[prjemian]

Let's implement reflection labels in a branch from 289-restore-reflections.

Originally posted by @prjemian in #292 (comment)

[prjemian]

Reflections are used/described in these files (and related tests):

file	number of instances of reflection text
configuration.py	40
diffract.py	20
sample.py	94
user.py	18
util.py	19

[prjemian]

This is not a trivial change to make. Due to the way reflections are handled now (libhkl keeps the defining list in its C memory), this could become a breaking change.

[prjemian]

Changing priority and milestone for this issue.