Merge loads optimization for network store IIDM impl

[geofjamg]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Performance improvement

What is the current behavior?

When use the mergeLoads option and the network store impl we have performance issue because network.getBusView().getBus(id) is poorly implemented.

What is the new behavior (if this is a feature change)?
As improving the performance of this method in the network store IIDM impl is not easy at all, we can workaround by caching bus in the dynawo integration code.

Does this PR introduce a breaking change or deprecate an API?

• The Breaking Change or Deprecated label has been added
• The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

Other information:

[sonarcloud]

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100.0% Coverage
0.0% Duplication