Unify and document loading Network objects

[GijsVermarien]

In order to load a network for interactive use, there exist a few pathways:

• Use Makerates interactively, and just use the output Network object (I suspect not many people do this at all)
• Use the get_reaction_networkfunction from uclchem.utils
• Load the network directly from species.csv and reactions.csv with get_network leaving the first argument empty and specifying path_to_species_file and path_to_reaction_file

This are mostly based on the old file based workflow, saving species/reactions to a csv during the process of Makerates. With the new interface and the ability to save to h5 rather than separate files, we might want to unify and streamline this experience and provide one interface.

Any thoughts @Marcus-Keil