Castep/VASP/LAMMPS data writers, including factory

include/tadah/mlip/dataset_readers/dataset_reader_selector.h

include/tadah/mlip/dataset_writers/castep_cell_writer.h

include/tadah/mlip/dataset_writers/dataset_writer.h

include/tadah/mlip/dataset_writers/dataset_writer_selector.h

include/tadah/mlip/dataset_writers/lammps_structure_writer.h

include/tadah/mlip/dataset_writers/vasp_poscar_writer.h

src/castep_cell_writer.cpp

src/dataset_writer_selector.cpp

src/lammps_structure_writer.cpp

src/vasp_poscar_writer.cpp