support SV, especially SV1_DESI_TARGET

[sbailey]

Updates to support the simulation chain with SV1_DESI_TARGET in prep for Survey Validation.

@michaelJwilson please try this "sv" branch with fastframe and report any problems to this PR and we'll get them fixed as part of this update.

[michaelJwilson]

Changes implemented so far look good. I didn't look at the complete code, so I can't testify further changes aren't required. I assume you want to deal with #493 on the sv branch first before merging?

[michaelJwilson]

Changes implemented so far look good. I didn't look at the complete code, so I can't testify further changes aren't required. Given #493 is done, seems good to merge.

The current implementation for fastframe and clobber will work, I was just wondering if you really want write_frame to default to overwrite without the option to change it.

[sbailey]

Let's punt the remaining fast frame clobber question to a future issue / PR / decision and merge this now. I think it's ok to have fastframe overwrite outputs when called. The original reason for wrap-fastframe not doing that was for "pick up where you left off" -- if a job timed out you could just rerun the exact same wrap-fastframe command and it would just continue with any remaining pixels, without crashing on pre-existing pixels nor wastefully regenerating them, and without requiring the user to update the list of requested healpix.