WIP: Module: Add `nucleic` keyword to MDTraj atom selection language
MDTraj has a powerful atom selection language, which allows users to select atoms based on a human-readable syntax. One of its features is the ability to select atoms that are part of a protein by using the keyword "protein". However, the same functionality didn't exist for nucleic acids. This module adds the ability to select atoms from nucleic acid residues using the keyword "nucleic."