Support having multiple active operational limits group

[SylvestreSakti]

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)
• A PR or issue has been opened in all impacted repositories (if any)

Does this PR already have an issue describing the problem?
No

What kind of change does this PR introduce?
powsybl-core PR powsybl/powsybl-core#3735 will introduce multiple operational limits groups. This current PR aims at supporting this new functionnality.

What is the current behavior?
Only one operational limits group can be selected

What is the new behavior (if this is a feature change)?
Multiple operational limits group can be selected. A new class LfLimitsGroup (which only contains a list of LfLimit objects) has been introduced to avoid using nested lists.

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

• Yes
• No

If yes, please check if the following requirements are fulfilled

• The Breaking Change or Deprecated label has been added
• The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)
For custom open-loadflow implementation using LfNetwork, the LfBranch API has been modified :
getLimits1() and getLimits2() return now a List<LfLimitsGroup> instead of a List<LfLimit>

Other information: