[Summary Issue] Improve package plots!

[RayStick]

Across the package, various updates are needed for the plotting functions. When you are ready to work on one, please convert the bullet points into a separate digestible issues that be addressed across multiple PRs.

I would like to see these changes implemented:

• Currently the unit tests for test-end_plot.R and test-ref_plot.R are pretty basic. Should consider using snapshot tests! Specifically, the rOpenSci guidance said: For testing your functions creating plots, we suggest using vdiffr, an extension of the testthat package that relies on testthat snapshot tests.
• Enhancement suggested by @BatoolMM - "Is there any value of enabling the user to customise it, it is created with ggplot - but not sure if there is a function of making it easily customisable by the user or adding it to doc." Consider plotly?
• Enhancements suggested by @RayStick - Think about what else we could plot in browseMetadata.R that are common features that would be helpful to summarise:
  • display which variables have missing descriptions
  • group variables by datatype
  • group by lookup table? (too similar to the actual mapMetadata.R outputs?)
  • use the DO NOT USE flag etc.