Clean up shared test functions, ensure coverage of edge cases

[Sparks29032]

• If any code is reused in tests, put it all in a function. If a particular function is shared across multiple test files, put it in the conftest.
  • Rethink names of functions (e.g. isfloat -> is_float, are_files_same -> files_same). This is to mirror numpy's all_close function naming.
• Think up edge cases and cover them. One important one: smear-pdf when the target and objective g(r) functions are on different grids. This has not been covered in our tests.
• Need tests for interfacing plotting and morphapp.

[Sparks29032]

Internal (not user-facing) change. Moved to 0.3.1.

[sbillinge]

@Sparks29032 please can you review what is left to do here. I see some linked merged PRs. If we summarize that, this will be easier to return to later. Then we can move the rest to 0.4.0

[Sparks29032]

I don't think anything is actually resolved in this issue.

The linked PRs were motivation for this issue since there was a pretty serious bug with smear-pdf that I noticed when playing around.

We will need to take some time for each morph and think of edge cases.

[sbillinge]

ok, I moved to 0.4.0