v0.2.0

• Added functionality to write atom groups into xtc and trr files using the XdrGroupWriter trait along with the XtcGroupWriter and TrrGroupWriter structures.
• Introduced an optional property chain to the Atom structure, allowing specification of the chain ID to which the atom belongs. This property is automatically loaded from pdb files containing chain identifiers and it can also be set manually using the Atom::with_chain function.
• When writing pdb files, if the chain property is set for the Atom, it is now written into the pdb file.
• A new keyword chain has been added to the Groan Selection Language (GSL) that allows to select atoms based on the chain they belong to.
• Made the constructors for AtomIterator and MutAtomIterator public.
• Implemented operations for ndx-writable and ndx-nonwritable groups, including System::group_make_writable, System::group_make_nonwritable, and System::group_names_writable.
• Performed small code refactors.
• Made minor documentation fixes.