geom_spoke: add a parameter that controls the unit for the radius

[ASmirnov-HORIS]

The radius of the spokes can be interpreted differently in different plots:

1. The radius units are bound to the axis units.
   Example:

   wind_data = {
       'x': [0, 0, 0, 1, 1, 1, 2, 2, 2],
       'y': [0, 1, 2, 0, 1, 2, 0, 1, 2],
       'speed': [0.84, 0.89, 0.92, 0.89, 0.94, 0.97, 0.92, 0.97, 0.99],
       'direction': [0.78, 0.72, 0.69, 0.84, 0.78, 0.75, 0.87, 0.81, 0.78],
   }
   
   ggplot(wind_data) + \
       geom_spoke(aes('x', 'y', radius='speed', angle='direction'),
                  arrow=arrow(type='closed', angle=15))

   
   This is exactly how Lets-Plot works now.

2. The radius has its own units.
   Example:

   volcano_data = {
       'lat': [13.60, 14.88],
       'lon': [41.80, 39.92],
       'eruptions_count': [3, 2],
   }
   
   ggplot(volcano_data, aes('lon', 'lat')) + \
       geom_livemap(zoom=8, location=[40.86, 14.24]) + \
       geom_point(size=5, color='red') + \
       geom_spoke(aes(radius='eruptions_count'), angle=0.79, color='red')

   
   Here we want to be able to change the length of the spokes arbitrarily using a scale (see also issue #1070).
   To do this, we need to be able to change the units of the radius.