Explorer: disable the geo controls when geoviews is not installed

[maximlt]

Addressing (a tiny) part of #1190 with a different take compared to #1200 whose goal is to cover all kind of exceptions the explorer might raise, while in this PR I focused on finding the best way to let them know they should install GeoViews to enable the geo controls.

I ended up trying to import geoviews on initialization and if that fails disabling all the geo controls:

The Checkbox widget has no tooltip so instead I used a Toggle with a label that is updated to indicate GeoViews is required.
Trying to instantiate an explorer by passing geo kwargs (e.g. df.explorer(geo=True)) will raise an import error.

[ahuang11]

Looks good to me! The only thing is tiles doesn't seem to work without geographic projections, but I think that's a separate issue.

e.g. need to call: lon_lat_to_easting_northing

import holoviews as hv

hv.extension("bokeh")

hv.element.tiles.EsriImagery() * hv.Points(
    hv.util.transform.lon_lat_to_easting_northing(-88, 40)
)

[maximlt]

@ahuang11 Isn't it that you need to provide a dataset that has coordinates already projected to easting/northing to use tiles? Not all geo datasets are in lat/lon so I'm not sure the explorer should perform the conversion by default.

[ahuang11]

Maybe an extra toggle in the future that allows conversion to easting northing? Or at the very least, a tooltip that elaborates on that so new users aren't confused.

[maximlt]

Yes I think that'd make sense.