Load topology from JSON file

[jan-stevens]

Adding support for loading molecule topologies from JSON files. For large and simple systems, JSON-based topology files are easily created and speed up the generation of the topology object tremendously. (i.e., for the 500Kb chromosome)

An initial idea for implementing this would be simply using the from_json function of the MetaMolecule class in order to add all the molecules iteratively to an empty topology object.