Attach ScaleBar to subcoordinate axis

[droumis]

Allow adding scale bars to individual subplot/subcoordinate_y ranges. The current implementation only allows for a scale bar to be positioned relative to the entire plot. This depends on progress in Bokeh: https://github.com/bokeh/bokeh/issues/13921

A possible API might be to use a new arg; something like scalebar_level="subcoordinate_y"

Code

```python import holoviews as hv import numpy as np hv.extension("bokeh") # scalebar_level="subcoordinate_y") common_opts = dict(subcoordinate_y=True, scalebar=True, scalebar_range='y', scalebar_unit=('cm', 'm'),) c1 = hv.Curve(np.random.rand(1000), label='c1').opts(color='lightblue', scalebar_location="left", scalebar_opts={'bar_length':.1, 'background_fill_alpha': .2, 'bar_line_color':'darkblue'}, **common_opts) c2 = hv.Curve(np.random.rand(1000), label='c2').opts(color='indianred', scalebar_location="bottom_right", scalebar_opts={'bar_length':.1, 'background_fill_alpha': .2, 'bar_line_color':'darkred'}, **common_opts) c3 = hv.Curve(np.random.rand(1000), label='c3').opts(color='plum', scalebar_location="top_left", scalebar_opts={'bar_length':.1, 'background_fill_alpha': .2, 'bar_line_color':'purple'}, **common_opts) c4 = hv.Curve(np.random.rand(1000), label='c4').opts(color='palegreen', scalebar_location="right", scalebar_opts={'bar_length':.1, 'background_fill_alpha': .2, 'bar_line_color':'darkgreen'}, **common_opts) curves = hv.Overlay(c1 * c2 * c3 * c4).opts(show_legend=False) curves.opts(width=800, height=600) ```

Arrows indicate where the scale bars should be after applying to subcoordinate level: