This function returns a cleaned-up (simplified) version of the protein
structure. Different parameters affect the behaviour of the function.
:param strip_water: Whether to remove water from the structure
:param canonicalize: Whether to strip off modifications of amino acids and map
them back to their parent standard amino acid, e.g. selenium methionine to
methionine.For more complex amino acids, where the relation between the
modified and the standard parent amino acid is not known, sidechain atoms
are removed. D-peptide-linking residues are completely removed as well.
:param remove_ligands: Whether to remove ligands from the structure
:return: a cleaned version of the entity