Make the explorer easier to use

[maximlt]

Fixes #764
Fixes #787

• Should the accessors be patched by default when hvplot.explorer is called? I think so. I changed my mind, less side effects are better, at least for a start.
• Should the explorer allow to switch the plotting backend? I'm not sure. Let's keep it simple for now.
• Discuss and extend the API
• Add tests
• Add docs. I suggest marking the explorer as experimental on its doc page.

@philippjfr in 87b07b4 I've removed the __new__ methods. They were causing __init__ to be called twice, which wasn't really an issue as far as I could tell but was for sure undesired. I found no way to fix that by keeping them, so replaced them by a factory class method from_data that I use in the explorer function. This is a breaking change, which might be fine given that the explorer has been released recently and never documented. If this breaking change is unwanted, I could add a warning when an explorer isn't instantiated with from_data, or keep digging to try to fix the issue with __new__.

[philippjfr]

Will probably look into the __new__ issue myself. Please remind me when this is otherwise ready.

[maximlt]

@philippjfr this is ready to be reviewed, if you want to have a look into __new__.

[maximlt]

Made a few changes after some discussion:

• Raise an error when the data type given to hvplot.explorer is a geodataframe or an xarray object. This is because I found their support not good enough to be shared.
• Similarly the Geo tab has been removed, it did not work quite well from what we've seen (e.g.Enhancement to Geo tab in explorer #810)
• renamed the repr method to plot_code, repr was misleading
• improved the docs in general

[hoxbro]

If #810 is not expected to be in the next release, can you add these changes to this PR?

[maximlt]

@philippjfr do you still want to have a look at the __new__ thing or can I merge?

[philippjfr]

Go ahead and merge, __new__ hackery is usually ugly and unexpected so I'm fine with the current approach.

[philippjfr]

Thanks for your great work here!