LfNetwork serialization

[flo-dup]

Describe the current behavior

Launching several loadflows on separaed environments needs the LfNetwork to be rebuild.

Describe the expected behavior

The LfNetwork can be serialized/deserialized so that it can be saved (and exchanged) and later reloaded.

Describe the motivation

This feature is motivated by massive launches of loadflows. This feature really makes sense only if the LfNetwork deserialization is significantly quicker than building it from the iidm network.

Extra Information

Note that deserializing the LfNetwork will need the iidm network aside, as some LfNetwork elements contain iidm objects. Therefore the iidm file loading cannot be avoided.

[flo-dup]

This issue is low priority, but as some preliminary work has been done by @CBiasuzzi the issue has been created to keep track of that work and the corresponding brainstorming.