Use DanglingLineBoundaryImpl of core instead of a special implementation

[olperr1]

Describe the current behavior

DanglingLineBoundaryImpl is redefined in network-store but the powsybl-core's implementation could be directly used instead.

Describe the expected behavior

Use powsybl-core's DanglingLineBoundaryImpl instead of redefining it.

Describe the motivation

Using the powsybl-core class will prevent having to reflect its changes in powsybl-network-store.

Extra Information

No response