Improve efficiency of local exceedance intensitry/impact and local return periods functions

[ValentinGebhart]

Changes proposed in this PR:
Changes address the methods Hazard.local_exceedance_intensity, Hazard.local_return_period, Impact.local_exceedance_impact, and Impact.local_return_period.

• The interpolation util function is now only called for centroids with nonzero intensity/impact.
• Intensities/Impacts are rounded to 3 significant digits, as 2 significant digits as before led to bad extrapolation behaviour.
• docstring was extented to explain how data are extrapolated.
• floating point problem in util.interpolate.round_to_sig_digits was fixed.
• The plotting functions Hazard.plot_rp_intensity and Impact.plot_rp_imp now directly call the corresponding functions above, and a warning is added about how to recover values that are computed with CLIMADA 5.0.0 or earlier.

Comments:

• The extrapolation method might still be improved in the future.

This PR fixes #1010

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[chahank]

Cool idea, I am not sure though that this should be an info logging. @emanuel-schmid : what would be the most elegant way to inform the users that something changed, without having logs all the time?

[chahank]

Could you make this calculous optional? Or what would be the way to refine the plot without having to recalculate these values (which can take quite long)? Is it using the util method instead?

[ValentinGebhart]

I think the best way is to do gdf, title, columns = hazard.local_exceedance_intensity() and u_plot.plot_from_gdf(gdf, title, columns). Then you can just change the plot options by repeating the second step, without recalculating the gdf. The plot_rp_intensity is just a wrapper around these two functions, which is why I asked if we want to keep it in the first place.

One way of not repeating the calculation here if one calls the function plot_rp_intensity twice would be to save the local exceedance gdf as an attribute of the Hazard object, but I don't think we want to do that?

[ValentinGebhart]

What about writing the info to better use the other two functions in the docstring, as you suggested in the issue? Is this enough?

[chahank]

Yeah, that would be a solution I think. For small hazards I guess the method is rather fast, and for lager ones there would the solution in the docstring.