fix issue that if retval units is None then raises TypeError which caused the formula dictionary loop to quick prematurely so that args and other formula attributes were not set - since args was [] this caused uncertainty wrapper to fail
also update pvpower demo to pvlib-0.4 - specifically update clearsky to new api, add formulas for Linke turbidity lookup and rearrange the formulas to resolve dependencies - also add tl to outputs
add a default method to import formulas if none are found - use inspect to test if module attribute is function and import all functions - assumes that if no formulas are preceded with f_ then all functions are formulas
retvalunits isNonethen raisesTypeErrorwhich caused the formula dictionary loop to quick prematurely so thatargsand other formula attributes were not set - sinceargswas[]this caused uncertainty wrapper to failoutputsf_then all functions are formulas