Can't fit data without 5.5 point

[BethanyRS]

Lines 675 and 676 in base.py

( else: # if min(obs_x) > 5, use 5.5 um lam = 5.5)

result in "ValueError: A value in x_new is below the interpolation range" when there isn't a point around 5.5 microns (e.g. if you're fitting a subset of data from 6-14 microns).