pvlib.pvsystem.sapm can't be used if only effective irradiance is known

[mattguttenberg]

Since direct irradiance and diffuse irradiance are used to calculate the effective irradiance, another input should be added for the effective irradiance so that if it is known, the calculation is bypassed. This will remove unnecessary error if the value is a known value or makes the function usable if the value is the only one that is known.

[wholmgren]

See #198.