Improve Ferret package (#17620)

* Ferret: Add missing dependency with curl.

* Ferret: Don't force using the static version of libgfortran.

* Ferret: Ensure Spack's compiler wrappers are used.

This allows properly setting the rpaths.

* Ferret: Add support for versions 7.3 to 7.6.

* Ferret: Add a variant to install Ferret standard datasets.

* Ferret: Define some useful runtime environnement variables.

* Ferret: Fix flake8.

Also add myself as a maintainer as suggested by @alalazo.