Feature/choice unsequa helper

[chahank]

Changes proposed in this PR:

• Addition of uncertainty helper methods variables: list of hazard, list of impact function sets, and hazard fraction
• This allows to pre-compute hazards or impact function sets from different sources from which one can then sample uniformly.

This PR addresses points in https://doi.org/10.5194/gmd-2021-437-RC1 and https://doi.org/10.5194/gmd-2021-437-CC1.

Pull Request Checklist

• Correct target branch selected (if unsure, select develop)
• Source branch up-to-date with target branch
• Docs updated
• Tests updated
• Tests passing
• No new linter issues

[simonameiler]

These modifications allow loading an ensemble of perturbed event hazard sets or impact functions into CLIMADA in addition to the existing functions of applying perturbations within the platform. With this update, users could do extensive uncertainty and sensitivity analyses of the hazard model or impact functions outside CLIMADA and then use the platform to combine the samples with the remaining components for impact calculations and perform the final calculations of sensitivity indices.

The uncertainty helper methods description is improved.