Refactor the NAD Layout API for nodes with fixed positions.

[So-Fras]

Describe the current behavior

In order to provide fixed Node positions for a layout algorithm, the user must first call Layout > setInitialNodePositions(...) with a map containing Node ids and their respective positions and then fill another set with the Node ids (thanks to the Layout > setNodesWithFixedPosition(...) function) set in order to prevent the algorithm from moving those nodes.

Describe the expected behavior

It would be clearer to have a function Layout > setNodesWithFixedPosition(...) directly taking a Map<String, Point> as an argument.

Describe the motivation

User-friendliness

Extra Information

No response