added chart option "unit" for special target units

[roe-dl]

I missed an option to set the target unit for an observation type in graphs.conf. So I would like to introduce a new option units that allows to set the unit individually for each chart. That comes in useful if you want to show graphs for summarized readings with an unit starting with "kilo" or "mega" for long time periods (month, year), but use the base unit for short time periods (day, week, may be). Using that option converts the values of the chart line it is specified for to the given target unit and sets the unit label appropriately. I hope that will be useful to other users, too.

Typical units for that would be watt_hour and kilowatt_hour or watt_hour_per_meter_squared and kilowatt_hour_per_meter_squared, respectively.

Example:

    [[sun]]
        ...
        aggregate_interval = 86400
        start_at_midnight = true
        [[[radiation]]]
            aggregate_type = energy_integral
            units = kilowatt_hour_per_meter_squared

[roe-dl]

Example: https://www.woellsdorf-wetter.de/graphs/?graph=month#sunshine

config:

    [[sunshine]]
        title = Sunshine Duration
        aggregate_type = sum
        aggregate_interval = 86400
        start_at_midnight = true
        [[[sunshine]]]
            type = column
            color = "#ffc83f"
            units = hour
            [[[[numberFormat]]]]
                decimals = 1

[roe-dl]

From weewx/weewx@77b00e5 I learnt that "unit" is used in singular in English, too. So I changed the word to singular.

[poblabs]

Can you also please update the charts wiki with this info?