label scale

[mkness]

As raised in Anna's repo, should also fix this in this test version of the code:

For stability we should have labels pivoting around the same order of magnitude. Ideally, all labels would be an object with a scaling (factor) and offset value. Currently there is an offset only, so values vary by a factor of ~ 100+ (i.e. the Teff scale is on the order of hundreds, the logg, [Fe/H] scale is on the order of ~1. This is not too bad but should be fixed for the general label case.

[davidwhogg]

My view is that there should be a label "object" which has attributes like "name" "offset" "scale" "range" and so on, and eventually "ln_prior". cc @annayqho