defined the SiteType enum and AtomArrangementItem dataclass to handle the coordinates and site types, ensuring validations.
AtomArrangement Class:
 The class initializes with an empty list of sites.
Add Method: Adds a coordinate to the arrangement with a site type.
 Retrieves a list of coordinates for a given index.
 Rounds coordinates to the nearest multiple of the given resolution.
Factory Methods: Methods for creating square and rectangular lattice arrangements.
Demonstrates creating a square lattice with specified boundary points and lattice constant.

fixes amazon-braket#969

I have cleaned up and properly formatted the code to enhance readability and maintainability. Here are the key changes made:
Added all necessary imports at the beginning of the code.
Defined a custom exception DiscretizationError.
Organized and added necessary data classes (LatticeGeometry, DiscretizationProperties, AtomArrangementItem) and enums (SiteType).
Implemented validation methods inside AtomArrangementItem to check coordinates and site types.
Added docstrings to methods for better understanding.
Implemented methods for adding coordinates, getting coordinate lists, iterating, getting length, and discretizing arrangements.
Added factory methods for creating different lattice structures (square, rectangular, decorated Bravais, honeycomb, and Bravais lattices).