Implement degree-valued trigonometric functions and standardise code according to Fortran2008

• Implement degree-valued trigonometric functions
  • Can be activated with -fdec-math for gfortran, but not available for nagfor
• Standardize by using int64 from iso_fortran_env instead of kind=8
  • kind=8 is not recognized by nagfor. integer(kind=8) -> integer(int64)
  • kind=4 is not recognized by nagfor. integer(kind=4) -> integer
• Real(kind=8) -> Real(kind=dp)
  • kind=8 is not recognized by nagfor
• Use newer get_command_argument() instead of getarg()
• call flush() -> flush()
  • Beginning with the Fortran 2003 standard, there is a FLUSH statement that should be preferred over the FLUSH intrinsic.
• Temp solution: sizeof(z(1,1,1)) -> kind(z(1,1,1))
  • sizeof is not recognised by nagfor