hvPlot Explorer issues

These things make the explorer unintuitive to use:

# Minor
- [x] Rot needs a better label
- [x] Clim could be renamed to Colorbar limits?
- [x] Tiles should be possible to activate without geo=True
- [x] "Live update" widget isn't aligned well

# Medium
- [x] Null value isn't visible in a Multiselect widget, so once a value has been selected, it's not intuitive how to go back to unselected (which is Command-click on Mac)
- [ ] Remove unsupported datashader aggregate option of count for gridded
- [ ] Color widget should be disabled for gridded, since it does not apply
- [ ] When showing an error, mention that the plot will not update properly until the error message is corrected

# Major

- [ ] By is unintuitive and often doesn't work with groupby (add a tooltip toexplain what it is, and/or switch to using groupby + a radio button to select overlay, layout, widget, or color, and/or enforce value on by needed for groupby or vice versa)

# TBD

- [x] Widget_location for HoloMap/DynamicMap uses up screen space, but holoviews pane does not support column
- [ ] Two time(s) in the fields list: 'time' and "['time']"
- [x] Groupby should be None instead
- [ ] Height / width doesn't do anything
- [x] Cartopy needs installed or else everything breaks silently; need to catch all such errors to avoid locking UI

# Discussion needed
- [ ] Frame wise / axis wise option

# Out of scope in hvPlot
- [ ] Infer color label from dataset (right now colorbar has no text label even though the dataset declares that dimension; explicitly filling in "Temp" works on clabel)
- [ ] Bokeh invalid tooltip (e.g. for "HoloViews .opts()" under Advanced, the tooltip disappears permanently after entering invalid input

Credits to @jbednar for discovering these.