Display a color bar when rasterize is enabled

[maximlt]

Fixes #777

Before #206 the cmap style was handled as follows, i.e. when datashader was involved it was set to the default linear colormap:

        if cmap is not None:
            style_opts['cmap'] = cmap
        elif self.rasterize or self.datashade:
            style_opts['cmap'] = self._default_cmaps['linear']

#206 made hvplot behave more closely to xarray.plot() by using a symmetric colormap if the data is found to be divergent. #206 replaced the code that set the colormap to linear, by setting the color bar, to True by default:

        if cmap is not None:
            style_opts['cmap'] = cmap
        elif self.rasterize or self.datashade:
            plot_opts['colorbar'] = plot_opts.get('colorbar', True)

This PR:

• moves up the logic that handles the datashaded path, as it seems it could be set up-front
• sets the color bar to True by default when rasterize=True, whatever the value of cmap. This fixes the related issue. I'm not entirely sure this is the correct behavior for all plot kinds.
• doesn't set the color bar when datashade=True as a color bar isn't relevant with this option.

[jbednar]

Seems ok to me, given that hvplot seems to default to using a colorbar whenever it's relevant, and it's certainly relevant when datashading! Presumably specifying cnorm='eq_hist' overrides the linear setting?

[maximlt]

Presumably specifying cnorm='eq_hist' overrides the linear setting?

Yes it does:

[philippjfr]

Perfect, fix looks good!