Measure control's `add_length_unit` function doesn't work

[louisstuart96]

Sample code from official documentation (https://ipyleaflet.readthedocs.io/en/latest/controls/measure_control.html):

from ipyleaflet import Map, MeasureControl, basemaps

m = Map(center=(43.0327, 6.0232), zoom=9, basemap=basemaps.OpenStreetMap.Mapnik)

measure = MeasureControl(
    position='bottomleft',
    active_color = 'orange',
    primary_length_unit = 'kilometers'
)
measure.completed_color = 'red'

measure.add_length_unit('yards', 1.09361, 4)
measure.secondary_length_unit = 'yards'

measure.add_area_unit('sqyards', 1.19599, 4)
measure.secondary_area_unit = 'sqyards'

m.add(measure)
m

Result shows: (Google colab with ipyleaflet v0.20):