pvlib/ivtools/sdm.py is currently rather lengthy at ~1350 lines of code. Near-term additions (#2212 and #2185) will bring it to ~2000. I suggest we split up sdm.py into submodules, similar to what was recently done in pvlib.spectrum (#2125).

Current and future functionality spans:

• estimating parameter values for various SDMs using various data sources
• converting parameter values from one SDM to another (WIP: Convert between CEC and PVsyst single diode models #2212)
• calculating derived values (PVsyst temp coeff)
• calculating SDE parameters (if we move the calcparams_XX functions here)

How should it be split up? As a starting point, one idea is to organize the parameter fitting functions according to the data source:

• sdm/_fit_datasheet.py: fit_cec_sam, fit_desoto
• sdm/_fit_ivcurves.py: fit_pvsyst_sandia, fit_desoto_sandia
• sdm/_fit_iec61853: Add method to fit PVsyst SDM to IEC 61853 data #2185
• sdm/_convert.py: WIP: Convert between CEC and PVsyst single diode models #2212
• sdm/_misc.py: pvsyst_temperature_coeff

This division has the advantage of keeping fit_pvsyst_sandia and fit_desoto_sandia together, which is nice because they share a lot of code.

Questions:

• Should the calcparams_XX functions be moved to pvlib.ivtools.sdm?
• Does this division accommodate future functionality additions?