Bombproofing readers plus some fixes

include/tadah/mlip/dataset_readers/castep_castep_reader.h

include/tadah/mlip/dataset_readers/castep_md_reader.h

include/tadah/mlip/dataset_readers/dataset_reader.h

include/tadah/mlip/dataset_readers/dataset_reader_selector.h

include/tadah/mlip/dataset_readers/vasp_outcar_reader.h

include/tadah/mlip/dataset_readers/vasp_vasprun_reader.h

include/tadah/mlip/structure.h

include/tadah/mlip/structure_db.h

src/castep_castep_reader.cpp

src/castep_md_reader.cpp

src/dataset_reader.cpp

src/dataset_reader_selector.cpp

src/structure.cpp

src/structure_db.cpp

src/vasp_outcar_reader.cpp

src/vasp_vasprun_reader.cpp

tests/test_atom.cpp

tests/test_dataset_readers.cpp

tests/test_structure.cpp