Less boilerplate for optional dependencies

[ocefpaf]

This PR reduces the amount of code when adding an optional dependency. I do not think we have anything like this outside folium/utilities.py.

I think we should follow this simpler pattern in the future.

[ocefpaf]

@BibMartin this is not aimed to close #241 per se. But let me know if you are OK with this as a strategy deal with optional dependencies.

[BibMartin]

@ocefpaf
I'm not fully okay with this strategy. I'd prefer if the buggy functions were isolated into a plugin.
For example, I guess ImageOverlay would need to be a plugin (or at least the array->image part), so that the plugins would be tagged numpy dependent, while there's no risk of facing a bug when using classical folium.

[ocefpaf]

I'm not fully okay with this strategy. I'd prefer if the buggy functions were isolated into a plugin.
For example, I guess ImageOverlay would need to be a plugin (or at least the array->image part), so that the plugins would be tagged numpy dependent, while there's no risk of facing a bug when using classical folium.

That is better. Still, even as isolated plugins, I'd rather we import inside the function and let the ImportError be raised, than suffering the risk of a rogue import somewhere triggering the optimal dependency.

We can close this, leave as-is, and open another issue to isolated things. Or we can merge this, ship v0.2.0 and work on isolating things in v0.2.1. Your call. I am fine either way.

[themiurgo]

I'm with @BibMartin for long-term strategy on this.

[ocefpaf]

I closed this. We'll follow with @BibMartin's strategy.