Change restart warning into info

If a user state RestartWrite = no, the code is drawing a warning:

** Warning:
**   Restart information will not be written.

coming from the code (src/grid/restart.F90)

      if (restart%skip) then
        message(1) = 'Restart information will not be written.'
        call messages_warning(1, namespace=namespace)
      end if  

I think that because this is a user choice, this does not make much sense that we have a warning here. We should probably convert this to a warning.

@martin.lueders @AlexBuccheri Would you agree to change this to messages_info?