Fixed layout type for NAD

[CBiasuzzi]

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?

feature

What is the current behavior?

What is the new behavior (if this is a feature change)?

Adds a FIXED type to the NadLayoutType list, to be able to generate a NAD with a fixed layout (through the powsybl-diagram's FixedLayout implementation). The NAD's fixed positions, encoded as JSON metadata (e.g., as a result of a previously computed NAD), can be specified in a new NadParameters' parameter 'metadata'. Diagram elements' positions that are not listed in the JSON metadata will be computed using the Force Layout.

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)

Other information:

Note that prerequisite to this PR is the powsybl-diagram's powsybl/powsybl-diagram#656