FAIRness: add to community registry and evaluate quality

[sadielbartholomew]

See the description in the equivalent issue on cfdm, NCAS-CMS/cfdm#131 for context. In the case of cfunits, a further recommendation we don't fulfil relative to cfdm is storage of the code in a dedicated registry, such as Zenodo as used to register cf-python and cfdm already. Overall, for cfunits the evaluation tool currently reports the following:

$ howfairis https://github.com/NCAS-CMS/cfunits
Checking compliance with fair-software.eu...
url: https://github.com/NCAS-CMS/cfunits
(1/5) repository
      ✓ has_open_repository
(2/5) license
      ✓ has_license
(3/5) registry
      × has_ascl_badge
      × has_bintray_badge
      ✓ has_conda_badge
      × has_cran_badge
      × has_crates_badge
      × has_maven_badge
      × has_npm_badge
      ✓ has_pypi_badge
      × has_rsd_badge
      × is_on_github_marketplace
(4/5) citation
      × has_citation_file
      × has_citationcff_file
      × has_codemeta_file
      × has_zenodo_badge
      × has_zenodo_metadata_file
(5/5) checklist
      × has_core_infrastructures_badge

Calculated compliance: ● ● ● ○ ○

It seems you have not yet added the fair-software.eu badge to your README.md. You can do so by pasting the following snippet:

[![fair-software.eu](https://img.shields.io/badge/fair--software.eu-%E2%97%8F%20%20%E2%97%8F%20%20%E2%97%8F%20%20%E2%97%8B%20%20%E2%97%8B-orange)](https://fair-software.eu)

and given we have a score of only 3/5 in this case I have held back on adding a badge to the README until the score is four or more.