improves transition to a new VL (explorer's NAD tab)

[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?

improves visual transition to a new VL (explorer's NAD tab). More evident when it takes some time to compute the NAD for a new VL, the NAD tab view, with the current NAD, was reset to its original state before starting the computation.
This update keeps the pan&zoom levels and the position of the nodes for the current NAD, until the NAD for the new VL is ready.

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: