Download and use ERA5 low and high veg cover fraction

[kmdeck]

Is your feature request related to a problem? Please describe.
ERA5 data includes a timeseries for high vegetation LAI and low vegetation LAI. We had been summing these, but they should be weighted by a coverage fraction. This can be downloaded from ERA5:https://cds.climate.copernicus.eu/datasets/reanalysis-era5-single-levels?tab=overview

Please link to any related issues/PRs.

Describe the solution you'd like

1. Download these fields.
2. These are independent of time, but when you download them, theyll have a time index. Remove this.
3. Add to the era5 clima artifact 2008 and 40 years OR make a new artifact
4. add to Climaland:

function prescribed_lai_era5(
    era5_lai_ncdata_path,
    era5_lai_cover_ncdata_path,
    surface_space,
    start_date;
    time_interpolation_method = LinearInterpolation(PeriodicCalendar()),
    regridder_type = :InterpolationsRegridder,
)
hv_cover = SpaceVaryingInput(era5_lai_cover_ncdata_path, surface_space, varname_hv)
lv_cover = SpaceVaryingInput(era5_lai_cover_ncdata_path, surface_space, varname_lv)
compose_function(lai_hv, lai_lv; hv_cover = hv_cover, lv_cover = lv_cover)
    return lai_hv* hv_cover + lai_lv * lv_cover # I dont think we need to normalize
end

    return TimeVaryingInput(
        era5_lai_ncdata_path,
        ["lai_hv", "lai_lv"],
        surface_space;
        start_date,
        regridder_type,
        method = time_interpolation_method,
        compose_function = compose_function,
    )
end

6. Compare timeseries of global mean LAI in 2008 from MODIS and ERA5 with this fix. they should be similar.

Describe alternatives you've considered
We also downloaded MODIS for 2008, but this is only one year. On the other hand, we already have 40 years of ERA5 data.

Additional context
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