Update nasa_power VARIABLE_MAP with additional parameters

Author: karlhillx

pvlib/iotools/nasa_power.py

@@ -18,6 +18,10 @@
     'T2M': 'temp_air',
     'WS2M': 'wind_speed_2m',
     'WS10M': 'wind_speed',
+    'T2MDEW': 'dew_point',
+    'T2MWET': 'dew_frost_point',
+    'TQV': 'precipitable_water',
+    'RH2M': 'relative_humidity',
     'ALLSKY_SRF_ALB': 'albedo',
 }