Draft: Resolve "Create interfaces for internal data structure"

Closes #60 (closed)

nucleus/principalNetworkDomain.go

 package nucleus
 
+import "code.fbi.h-da.de/cocsn/gosdn/nucleus/interfaces"
+
 type pndImplementation struct {
+	name         string
+	sbiContainer []interfaces.SouthboundInterface
+}
+
+//NewPNDImplementation creates a Principle Network Domain
+func NewPNDImplementation(name string, sbi ...interfaces.SouthboundInterface) interfaces.PrincipalNetworkDomain {
+	return &pndImplementation{
+		name:         name,
+		sbiContainer: sbi,
+	}
 }
 
 // Interface satisfaction