More usage of the tdfield_t data type, in the oct module.

Also, substituted save_tdfield for tdfield_save.
Eliminated a couple of unused routines.
Now dt_propagation receives thd tdfield_t data type, instead of the array.
Added a copy procedure for the tdfield_t data type.
Added tdfileld_fluence. Eliminated ref_tdfield, now unnecessary.
Moved the Fourier transform routines to the tdfield_m module.
Added one routine to multiply fields by some constant.