Method of specifying windSpeed or windGust for producing the stacked windrose is very user unfriendly

[Gary Roderick]

At the moment to change from using windGust to windSpeed as the source for the windrose plot the user must edit skin.conf (we are trying to avoid this) or add an extensive override in weewx.conf. It should be such that the user simply changes one (existing) setting in weewx.conf.

Also, the change of data source is made by changing a [[windSpeed]] stanza name, it should be done via a setting not a stanza name