Institute a --mimic-version kwarg instead of needing the alt-MTLs to switch code versions.

[geordie666]

In version 3.4.2 of desitarget, we updated the LyA quasar decisions to allow ELG targets that turned out to be consistent with being higher-z quasars to be re-observed as if they were LyA quasars.

The way this change is currently being handled in the alt-MTLs is to switch in the appropriate version of desitarget.

But at some point we will likely have to deprecate earlier versions of desitarget. So, we should instead provide something like a --mimic-version kwarg that runs the earlier MTL logic without the need to switch code versions.