Resolve "Unimplemented methods of PrincipalNetworkDomainAdapter"

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Closes #172 (closed)

northbound/client/sbi.go

+package client
+
+import (
+	spb "code.fbi.h-da.de/danet/api/go/gosdn/southbound"
+	"google.golang.org/grpc"
+)
+
+// SbiClient returns a client for the gRPC SBI service. It takes
+// the address of the gRPC endpoint and optional grpc.DialOption
+// as argument
+func SbiClient(addr string, opts ...grpc.DialOption) (spb.SbiServiceClient, error) {
+	conn, err := grpc.Dial(addr, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return spb.NewSbiServiceClient(conn), nil
+}