Fixed block data 2

[michlv]

• describe missing bits in fixed_block.display_3
• add rain_factor and wind_factor to fixed_block
• add date for uv and illuminance max values
• Make code dealing with top nibble more generic removing need for WSFloat.wind_gust and WSFloat.wind_ave as specialized methods
• add missing 4 top level bits for max values for rain 'month' and 'total'

[jim-easterbrook]

Thanks.