handle NaN as fillvalue for forcing fields

[mickaelaccensi]

Describe the bug
if NaN is used as FillValue in netcdf forcing field, ww3_prnc will not be able to handle it correctly.

Correction in ww3_prnc should be :

  USE, INTRINSIC :: ieee_arithmetic

IF ( (myvariable.EQ.FILLVALUE) .OR. ( IEEE_IS_NAN(myvariable) ) THEN