IDatafileInfo.GetData now async, along with other APIs

Added

• readme field on datafile root elements (gamesystem and catalogue) in code
  and in 2.03 schema (#115).

Changed

• In WarHub.ArmouryModel.ProjectModel.IDatafileInfo interface
  SourceNode? GetData() changed to async Task<SourceNode?> GetDataAsync();
  this also results in some APIs changing to be async as well, especially in
  Workspaces.BattleScribe namespace (#117).