Resolve "Add configuration to output dict in case of multiple configurations"

Closes #12 (closed)

Things done in this MR (as explained in the comments in #12 (closed)):

• Split NetworkLinker into NetworkBuilder and NetworkLinker. The builder only creates an empty network and the linker fills it with nodes and connections.
• The default builders and linkers within netconf_generator are changed such that there are only two default builders (ComponentBuilder and NetworkBuilder) and one default linker (NetworkLinker).
• netconf_generator now yields a dictionary with outputs of all the builders and a dictionary with the configuration parameters.

(Note: the changes to UPDATE functionality done within !17 (merged) are not reflected here yet. Based on which MR gets merged first, I will then include the necessary changes in tests in the other MR.)