Update nasa_power VARIABLE_MAP

[AdamRJensen]

The following variables are currently being mapped when using pvlib.iotoolss.get_nasa_power.

pvlib-python/pvlib/iotools/nasa_power.py

Lines 13 to 21 in 7198fe6

	VARIABLE_MAP = {
	'ALLSKY_SFC_SW_DWN': 'ghi',
	'ALLSKY_SFC_SW_DIFF': 'dhi',
	'ALLSKY_SFC_SW_DNI': 'dni',
	'CLRSKY_SFC_SW_DWN': 'ghi_clear',
	'T2M': 'temp_air',
	'WS2M': 'wind_speed_2m',
	'WS10M': 'wind_speed',
	}

I suggest that the following parameters be added:

'ALLSKY_SFC_LW_DWN': 'longwave_down',
'ALLSKY_TOA_SW_DWN': 'ghi_extra',   # or is it dni_extra?
'CLRSKY_DIFF': 'dhi_clear',
'PS': 'pressure', # check the units
'RH2M': 'relative_humidity',
'SRF_ALB': 'albedo',
'T2MDEW'  # dew point temperature
'T2MWET': # dew/frost point at 2 meters
'TQV': 'precipitable_water',  # check units

I used the following parameter list.

• Check that each of the parameters work