Risk Trajectory Split 3 : Impact Computation Strategies

[spjuhel]

As the Risk trajectory PR is too substantial, this is a third split.

This PR implements the concept of impact computation strategies.
An impact computation strategy defines what underlying process to follow to get from (exp,haz,vul) triplet to an impact.

The idea is to ease the extension of the trajectories to also allow non ImpactCalc based impacts (think network, unsequa, etc.)

Currently, it really is just a wrapper around ImpactCalc().

It has proved helpful in tests/prototypes of risk trajectory / option appraisal to, for instance:

• "custom" compute impacts on polygons / lines
• pre-assign centroid or not

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
• CLIMADA Reviewer Checklist passed
• Tests passing
• No new linter issues

[spjuhel]

Although the class does not need Snapshot, the unit tests do, so they will fail until #1197 is merged.