Need Tests for RHE

[GillySpace27]

Provide a general description of the issue or problem.

The sunkit-image.radial.rhef filter and its helper functions need more testing. We can discuss here which tests are required. The filter is associated with PR #205

Tests needed for RHEF:

> With and without bins given

> Various inputs for Upsilon

> plotting smaps natively

> Doing upsilon on a map that doesn't have RHE on it

> Make a notebook as an example of usage

[Cadair]

This should be a good starter issue, please make sure you can run the test suite locally if you start working on this.