[WIP] adds to the explorer's NAD tab an expand and remove node feature

[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?

What kind of change does this PR introduce?

adds a new feature that allows to remove and expand the NAD on a diagram's node, in the explorer's NAD tab

What is the current behavior?

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

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:

Please note that this feature currently needs:

• pypowsybl > v1.9.0 ( a version that includes the function get_network_area_diagram with the additional parameter fixed_positions )

The version in this PR shows a buttons bar in the NAD tab; the semantic of the mouse select action (SHIFT+CLICK on a node) in the NAD depends on the buttons bar current selection (go to VL/expand/remove).

An improved (and more recent) version, with a context popup menu on the NAD's VL nodes with the options (go to VL/expand/remove) is available in the PR #44