Default colourbar `extend='max'` when minimum level or value is zero

cf-plot has an 'extend' keyword option to cbar, levs and to specific functions to enable or disable extension markers on each end of the displayed colour bar, which is set by default to be enabled at both ends:

https://github.com/NCAS-CMS/cf-plot/blob/a325942415561e5a22773715ff3946e1416094f7/cfplot/cfplot.py#L8618-L8619

which produces colour bar ends like this (see red annotations):

However, in practice I have seen people show plots in presentations where the values plotted start at zero where the extension marker is still at the lower end. This made me think that we should update our default behaviour.

Namely, given very often when data values begin at 0(.0) that means the data in question doesn't make physical sense as a negative value, e.g. for a ratio or fraction such as albedo, for a vector magnitude such as wind magnitude, or for a scalar quantity such as mass which has no meaning in negative terms.

In these cases by default we still show the extension markers on the lower end, which is misleading data visualisation (and as much as the user should change this on the plot to correct it, often I have seen this hasn't been done - maybe they didn't have the time, or weren't aware that cf-plot allows one to configure this). In these cases, the colour bar should have extend='max' as the default behaviour, so that only the colour bar upper end gets the extension marker end:

(Note this would only be default behaviour, and can still be overridden by the user in case the range of possible values does extend below zero and they just happen to want to show the levels beginning with zero.)