Add format string corresponding to list-directed I/O

[Beliavsky]

I suggest that format string "(LD)" be added that corresponds to list-directed READ and WRITE in READ and WRITE statements. Having such a format would allow read (unit,fmt) and write (unit,fmt) to handle both formatted and list-directed output, removing the need for if blocks such as

if (format_present) then
   write (unit,fmt) a,b
else 
   write (unit,*) a,b
end if

The idea was proposed and discussed here, where Ron Shephard explains how it would simplify reading data. The "(LD)" syntax was suggested by Bill Long.