Risk Trajectory Split 1 : Snapshots

[spjuhel]

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

This PR implements Snapshots as the foundational object for risk trajectories.

These objects regroup Exposures, Hazard and ImpactFuncSet instances for a specific date.

The rationale is to provide an immutable snapshot of risk. As such, Exposures, Hazard, ImpactFuncSet and date inputs are deep-copied, and "protected" behind read-only properties.

Likewise, to avoid any ambiguity regarding adaptation, i.e., as a user, if I provide an adaptation measure, should I give the base (Exposures, Hazard, ImpactFuncSet) triplet to which the measure is applied ? Or the triplet where the measure as already been applied, a measure is always None and can only be set through apply_measure() which returns a new object (with the measure set).

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