change eta_m to module_efficiency

[wholmgren]

temperature.noct_sam uses eta_m_ref to describe the module efficiency at reference conditions and temperature.pvsyst_cell uses eta_m to describe the module efficiency generically.

Just calling both of these module_efficiency would make the function signatures easily understandable by many more people. I'd be ok with module_efficiency_ref but I don't think that precision is very important.

I skimmed pvterms and didn't see a suggestion for this quantity.

temperature.noct_sam has not yet been released and it's just a positional argument, so changing the name is trivial. temperature.pvsyst_cell would need a deprecation cycle.

Originally discussed in #1177 (comment)

Assignment of milestone indicates that we will act on this or close it forever before 0.9 is released.

[cwhanse]

I support this change.