Split up pvlib.ivtools.sdm into a subpackage organized by SDM

[kandersolar]

• Closes Proposal: split up pvlib/ivtools/sdm.py #2252
• 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.

This PR splits pvlib.ivtools.sdm into a subpackage with the following structure:

• ivtools/sdm/cec.py: fit_cec_sam
• ivtools/sdm/desoto.py: fit_desoto, fit_desoto_sandia
• ivtools/sdm/pvsyst.py: fit_pvsyst_sandia, pvsyst_temperature_coeff
• ivtools/sdm/_fit_desoto_pvsyst_sandia.py: helper functions for fit_pvsyst_sandia and fit_desoto_sandia

[cwhanse]

The reorganization looks good to me.