Small typo in irradiance.py

[bba-afk]

The 'klucher' model is named 'klutcher' in the irradiance.py model selection (line 379).

[wholmgren]

Thanks for pointing this out. Would you like to make a pull request with the fix? I think that the last part of the IPython notebook irradiance.ipynb may also need to be updated, though I have not checked.