SAM models: in dynamic_binary_number calculation, use a different interpolation variable besides redshift

[lzkelley]

Invalid bins have z=-1. Interpolating between these values and valid bins thus leads to spurious results (or at least results dependent on the arbitrary choice of negative redshift value). We should use something that is continuous past the age of the universe, i.e. time or scale-factor instead, so that we can more meaningfully continue to interpolate across.

The current implementation should be fairly conservative, so this is not expected to be a notable issue currently.