added lots of logging statements, why is it not interpolating?

Interpolation isn't always happening properly - sometimes the output
flux is just the nearest grid point
fixed interpolation problem - with 2 spectra left, bug ensued

When there were only two spectra to be interpolated, the code had
a bug that meant it wasn't actually interpolating, just grabbing the spectra
for that grid point.

Also I had mixed up selecting the upper and lower values for interpolation
that's been changed