Implement lead sites interface
This is an idea that originated from a discussion of !267.
builder.BuilderLeadwith an attribute
added_sites, that keeps track of which sites were added to the system (right now those very sites are returned by
- (Optional) ensure that deleting a lead also deletes those sites.
lead_sites(in addition to
lead_interfaces). This is a sorted array of sites (integers) that were added to the system and that belong to a lead.
This extension is also relevant for plotting.