Add Differentia package to index

[niccolozanotti]

This PR adds https://github.com/Nicholaswogan/Differentia to the package index.

Required

• Relevance - This is a package allowing for derivatives, gradients and Jacobians of Fortran functions/subroutines to be calculated using Forward Automatic Differentiation
• Maturity - the code is stable and the repo is actively developed.
  Availability - the source is freely available on GitHub.
• Open source - the package is licensed under MIT License.
• Uniqueness - this is an original repo, not a fork.
• README - this exists, clearly states the purpose, and contains information on the steps required to install, as well as a link to build the code and run the tests

Recommended

• Tests: the code is well tested, with tests being easy to run
• FPM: supports installation via fpm via fpm.toml

[jwallwork23]

Hi @ivan-pi, please could you re-review this?

[henilp105]

Thanks @niccolozanotti , LGTM!