Support dir2=auto

[tkittel]

As mentioned on #155, it might be useful to support dir2=any or dir2=auto (or even just allowing dir2 to be emitted although that is likely misleading). For cases where you only care about the primary orientation.

Discussed in #155

^{Originally posted by dddijulio November 23, 2023}
When using single crystals, we always need to specify dir1 and dir2 What if I don't care about the secondary orientation?