The internal structural object validate_models() method was being called once for each structure
when assembling the atomic coordinates. This resulted in the _translate() internal structural
object method, which converts all input data to formatted strings, being called hundreds of millions
of times. The problem was in lib.structure.internal.coordinates.assemble_atomic_coordinates(), in
that the one_letter_codes() method, which calls validate_models(), was called for each molecule
encountered. The solution was not to validate models in one_letter_codes().
