Unimplemented C functions are disabled in Fortran interfaces (system)

Some interfaces are missing in the C library and cause errors at link-time:

../../src/.libs/libescdff.so: error: undefined reference to 'escdf_system_write_metadata'
../../src/.libs/libescdff.so: error: undefined reference to 'escdf_system_write_magnetic_moment_directions'
../../src/.libs/libescdff.so: error: undefined reference to 'escdf_system_set_spacegroup_3d_number'
../../src/.libs/libescdff.so: error: undefined reference to 'escdf_system_read_magnetic_moment_directions'
../../src/.libs/libescdff.so: error: undefined reference to 'escdf_system_get_spacegroup_3d_number'

They have been disabled until the implementation becomes available.