Create an association between the PhysicalZone and Network. #55
Description
In addition to the System asset, we probably want to have Networks associated with particular PhysicalZone assets. This was suggested by @jesajx in #39 and it fits the scenarios presented in the Hardware Additions technique in MITRE, where both networks and systems are exposed to rogue devices once physical access to a particular area is obtained.
@jesajx also suggested that we allow a System to be present in multiple physicalZones which seems like a reasonable design choice.