logical flow in ft_read_xxx to support the plugin of new file formats

ft_read_header.m

hdr = ft_read_header(filename, varargin)

filetype = ft_getopt(varargin, 'filetype');
if isempty(filetype)
  filetype = ft_filetype(filename);
end

switch filetype
case 'aaa':
  % do whatever it used to be doing

case 'bbb':
  % do whatever it used to be doing

otherwise:
  % try adding fieldtrip/external/"filetype" to the path
  % it should contain the function read_header_"filetype" and that function should have the same API as ft_read_header
  if ft_hastoolbox(filetype, 3)
    funname = sprintf("read_header_%s", filetype);
    hdr = feval(funname, filename, varargin)
  end
  
end % switch