Resolve "Refactor how PNDs are implemented in the code"

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 #243

controller/nucleus/networkElement.go

@@ -282,6 +282,14 @@ func (n *CommonNetworkElement) MarshalJSON() ([]byte, error) {
 		sbiUUID = n.sbi.ID()
 	}
 
+	var pndUUID uuid.UUID
+
+	if n.PndID == uuid.Nil {
+		pndUUID = uuid.UUID{}
+	} else {
+		pndUUID = n.PndID
+	}
+
 	modelAsString, err := ygot.EmitJSON(n.Model, n.getYgotEmitJSONConfig())
 	if err != nil {
 		return []byte{}, err
@@ -297,6 +305,7 @@ func (n *CommonNetworkElement) MarshalJSON() ([]byte, error) {
 		TransportOptionType spb.Type  `json:"transport_option"`
 		SBI                 uuid.UUID `json:"sbi,omitempty"`
 		Model               string    `bson:"model,omitempty"`
+		PndID               uuid.UUID `json:"pnd_id,omitempty"`
 	}{
 		ID:                  n.ID(),
 		Name:                n.Name(),
@@ -307,6 +316,7 @@ func (n *CommonNetworkElement) MarshalJSON() ([]byte, error) {
 		TransportOptionType: transportOptionType,
 		SBI:                 sbiUUID,
 		Model:               modelAsString,
+		PndID:               pndUUID,
 	})
 }
 
@@ -350,6 +360,7 @@ func (n *CommonNetworkElement) MarshalBSON() ([]byte, error) {
 		TransportOptionType spb.Type `bson:"transport_option"`
 		SBI                 string   `bson:"sbi,omitempty"`
 		Model               string   `bson:"model,omitempty"`
+		PndID               string   `bson:"pnd_id,omitempty"`
 	}{
 		ID:                  n.ID().String(),
 		Name:                n.Name(),
@@ -360,6 +371,7 @@ func (n *CommonNetworkElement) MarshalBSON() ([]byte, error) {
 		TransportOptionType: transportOptionType,
 		SBI:                 n.sbi.ID().String(),
 		Model:               modelAsString,
+		PndID:               n.PndID.String(),
 	})
 }