Big initial commit.

commented out all code in init.
consolidated air mass calculation functions into one module.
consolidated clear sky functions into one module.
small documentation functions to each of those functions.
added critical floating point division fix to ineichin cs model.
added path check to linke turbidity look up table.
some progress towards PEP 8 compliance.
added light logging to these modules.
removed the ridiculous input data checks.
returning a single dataframe instead of tuples of single column dataframes.
the above change will probably break a lot of other modules.
replaced nested if else with elif