added universe plot tests to catch color=None and axis values

[shimwell]

Description

While plotting some geometry I kept getting an empty legend, I had set legend=True so I wasn't sure why it was blank 😕

Looking through the universe.plot source code I noticed that when setting legend to True we also have to set the colours. However the check for colors was not actually being triggered as plot.colors defaults to {}

This PR fixes the logic so that passing in legend=True without setting colors now triggers a ValueError if colors is not set as well as originally intended.

I've added a test to safe guard this feature and while in the universe plot unit test I also added some asserts to check the axis labels are as expected ✔️

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
  - [x] I have run clang-format on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
  - [x] I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks @shimwell!