[DRAFT] results comparison

_This is a draft epic which needs to be further developed_
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When analyzing the result of a decision model (e.g. ID), we may want to compare (at least) 2 different situations.
This is typical for value of X analysis, where X can stand for information, learning, control, flexibility and maybe others.

In such cases, we may want to compare 2 different structures represented by 2 different IDs with each others. Typically, one of the 2 structures is the base project.

The comparison may point differences in:
- MEU
- policies
- parameter sensitivity analysis

While traditional value of X looks at the MEU difference (ie. MEU(modified_ID) - MEU(base_ID)), we may want also to look at what happens to strategies and how robust they are (e.g. we may want to run SA on probabilities for EVSI).

This means we would need:
- Have the possibility to have 2 IDs in the project (the base one and the investigated one)
- Have the possibility to solve and analyse both IDs
- Have the possibility to easily visualise the effects of changes from base ID to modified ID (eventually on the same tab)
- Have the possibility to document the comparison for reporting

No files, tests, or entry points are identified. Begin by clarifying the comparison scope and locating how project IDs are currently solved and analysed; done means two IDs can be compared across MEU, policies, sensitivity analysis, visualisation, and reporting.

Written by the indexing model from the issue text.