refactor: relocate `diffpy.utils.wx`?

[ycexiao]

Problem

The issue comes from #347 (comment)

diffpy.utils.wx is used internally in the package, and it is now located in src/.

We want to look at where in the diffpy universe these internal used packages are being used and decide if it is better to move them elsewhere.

Proposed solution

Relocate diffpy.utils.wx to somewhere else?

[sbillinge]

@cadenmyers13 we could delay this to later. I am not sure it is that important. Let's not close this though.