Add direct irradiance to VARIABLE_MAP in get_era5 iotools function

[AdamRJensen]

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Direct irradiance has recently been added to the parameters supported by the ERA5 time series API.

The "fdir" parameter, also known as "total_sky_direct_solar_radiation_at_surface" is on a HORIZONTAL surface as described here.

Beam horizontal irradiance (BHI) is an accepted standard pvlib term, see nomenclature and is thus added to the variable map.