Implement lead sites interface
This is an idea that originated from a discussion of !267 (merged).
- Extend
builder.BuilderLead
with an attributeadded_sites
, that keeps track of which sites were added to the system (right now those very sites are returned byattach_lead
. - (Optional) ensure that deleting a lead also deletes those sites.
- Extend
system.FiniteSystem
withlead_sites
(in addition toleads
andlead_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.