fixed compile error for gcc 11.4

[pletzer]

With gcc 11.4 I get the following error:

src/seapodym_coupled.cpp: In function ‘void verify_identifier_string2(char*)’:
src/seapodym_coupled.cpp:621:29: error: cannot call member function ‘DF_FILE* gradient_structure::get_fp()’ without object
621 | gradient_structure::get_fp()->fread(str,num_bytes);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~^~
src/seapodym_coupled.cpp: In function ‘int save_identifier_string2(char*)’:
src/seapodym_coupled.cpp:646:29: error: cannot call member function ‘DF_FILE* gradient_structure::get_fp()’ without object
646 | gradient_structure::get_fp()->fwrite(str,length);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~^~
src/seapodym_coupled.cpp: In function ‘void save_long_int_value(long unsigned int)’:
src/seapodym_coupled.cpp:654:29: error: cannot call member function ‘DF_FILE* gradient_structure::get_fp()’ without object
654 | gradient_structure::get_fp()->fwrite(y,num_bytes);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~^~
src/seapodym_coupled.cpp: In function ‘long unsigned int restore_long_int_value()’:
src/seapodym_coupled.cpp:661:29: error: cannot call member function ‘DF_FILE* gradient_structure::get_fp()’ without object
661 | gradient_structure::get_fp()->fread(&tmpout,num_bytes);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~^~
make: *** [Makefile:151: objs/seapodym_coupled.o] Error 1
(

I think gradient_structure::get_fp() should be replace with gradient_structure().get_fp()