CM068: National Grid Legal Separation - consequential changes to reference NGESO in STC Section D and to provide clarity that only Transmission Owners create Transmission Investment Plans

This proposal seeks to modify the System Operator Transmission Owner Code (STC) to reflect the creation of a new National Grid Electricity System Operator (NGESO) that is legally separated from National Grid Electricity Transmission Limited.  References to NGET have been replaced by NGESO, where relevant, throughout the STC Section D in order to ensure the System Operator and Transmission Owner obligations are clear. It also removes the references in Section D, Part One for NGET Transmission Investment Plans.