Use IIDM extensions to store TieLine half lines

[zamarrenolm]

When merging two lines that are connected at a boundary node a TieLine is created. Use the IIDM extension mechanism to store the half lines corresponding to the original data.

[zamarrenolm]

The TieLine class will be removed in future version of IIDM. It will replaced with a regular Line class plus a MergedXNode extension.