compiler error for gcc 14

[pletzer]

These changes fix a compiler error when using gcc 14

src/seapodym_cohort.cpp:267:29: error: cannot call member function ‘DF_FILE* gradient_structure::get_fp()’ without object
267 | gradient_structure::get_fp()->fread(str,num_bytes);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~^~

The solution is to instantiate gradient_structure()

gradient_structure().get_fp()->fread(&tmpout,num_bytes);

Thanks for updating