#2812 Script should have caught error with findex use

[Jeremy Faden]

I had a script with a bug which could have been caught with some sanity checks. Something like I had

ff= findex(...)
ds= interpolate(ff,data)

when it should have been

ff= findex(...)
ds= interpolate(data,ff)

Other codes flag this sort of error...

[Jeremy Faden]

I wonder if a check for a bundle in findex would work. I'd need to show that findex can never be a bundle. I'm putting in a warning now (in interpolate), but I think this needs to be shown.