pvlib.ivtools.sdm.fit_desoto_sandia has incorrect sign for returned dEgdT

fit_desoto_sandia incorrectly returns +0.0002677 instead of -0.0002677 for dEgdT:

Lines 874 to 886 in 891778c

    
           dEgdT = 0.0002677 
        
           x_for_io = const['q'] / const['k'] * ( 
        
               1. / tok - 1. / tck[u] + dEgdT * (tc[u] - const['T0']) / tck[u]) 
        
           # Estimate R_sh_ref 
        
           nans = np.isnan(rsh) 
        
           x = const['E0'] / ee[np.logical_and(u, ee > 400, ~nans)] 
        
           y = rsh[np.logical_and(u, ee > 400, ~nans)] 
        
           new_x = sm.add_constant(x) 
        
           beta = sm.RLM(y, new_x).fit() 
        
           R_sh_ref = beta.params[1] 
        
           params['dEgdT'] = dEgdT

That value is used both for the returned value and in the equations that compute other SDM parameters. I've not inspected the math to see whether the sign should be fixed in both places, or only for the returned value.

Noticed in #2315.

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	dEgdT = 0.0002677
	x_for_io = const['q'] / const['k'] * (
	1. / tok - 1. / tck[u] + dEgdT * (tc[u] - const['T0']) / tck[u])

	# Estimate R_sh_ref
	nans = np.isnan(rsh)
	x = const['E0'] / ee[np.logical_and(u, ee > 400, ~nans)]
	y = rsh[np.logical_and(u, ee > 400, ~nans)]
	new_x = sm.add_constant(x)
	beta = sm.RLM(y, new_x).fit()
	R_sh_ref = beta.params[1]

	params['dEgdT'] = dEgdT

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