Improvements in Structure Generation, File Handling and Documentation.

[hongyi-zhao]

• Removed the extra spaces at the end of the first few lines when the write_POSCAR function writes the POSCAR file.
• Adopted Lattice and Poscar classes from pymatgen for streamlined structure object creation and POSCAR file management.
• Offered alternative methods to write POSCAR files, eliminating the need for intermediate file handling.
• Modified get_sites_elements function to enable the extraction of fractional coordinates and element symbols from a structure's primitive cell.
• Refactored super_cell function's parameters from Atoms to atoms to enforce consistency in naming conventions across the code.
• Adjusted the return statement order in super_cell from atoms, elements, lattice to lattice, atoms, elements, aligning with logical structuring of the input parameters order.
• Enhanced overall documentation clarity, including the explanation of the RBT acronym in the README.

[jinlhr542]

Many thanks to hongyi for his contribution