WIP: Convert between CEC and PVsyst single diode models

[cwhanse]

• Closes Add converters between single diode models #1935
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Primary reference is in review. Feedback on function naming and signature is welcome.

[mikofski]

Mesh grid?

[adriesse]

Feedback on function naming and signature is welcome.

I guess one might discuss whether you are converting models or sets of parameters.

[cwhanse]

Test failure for ubuntu-latest, 3.8, conda, -min is because that environment installs scipy=1.6.0, which doesn't allow for constraints when using Nelder-Mead, so the optimizer gets a different, unsatisfactory, solution.

[echedey-ls]

I guess one might discuss whether you are converting models or sets of parameters.

@adriesse , do you have any proposal in mind? For example, cec_params_to_pvsyst, params[_from]_pvsyst_to_cec [params_pvsyst2cec]. I don't really have a strong preference on wording.

scipy=1.6.0, which doesn't allow for constraints when using Nelder-Mead

scipy=1.7.0 is the first version to support it, and is also supported in Py3.9 (taking into account upcoming drop of Py3.8 support). I would edit this requirement for future >=Py3.9 release (#2213).

Source of scipy docs on nelder-mead; scipy 1.6.3 bumps to 1.7.0:

• 1.6.3 https://docs.scipy.org/doc/scipy-1.6.3/reference/optimize.minimize-neldermead.html?highlight=nelder%20mead
• 1.7.0 https://docs.scipy.org/doc/scipy-1.7.0/reference/reference/optimize.minimize-neldermead.html

[mikofski]

Is the simplex solver required? Why not try Powell instead?