Convert String name of the residue to an enumerate
In the Residue struct, it could be fine if the name of the residue is an enumerate. A function could convert the string to an enumerate and vice versa.
It could be used to find the type of the residue. In PDB files, a residue is a subunit of a chain and can be a Amino Acid, a Nucleic Acid, a lipid, a molecular or etc ... This could be used in fasta_seq and a lot of other functions.