The internal structural object validate_models() method was being called once for each structure
via the selection() method prior performing the translations, and once prior to performing the
rotations, for creating the atomic selection object. This resulted in the _translate() internal
structural object method, which converts all input data to formatted strings, being called hundreds
of millions of times. Therefore selection() method no longer calls validate_models().
This may speed up quite a number of internal structure object methods when large numbers of
structures are present.
