Added initial model "cache".
Calculate target v_d as the 95 percentile of the v_d distribution (default).
Bugs in v_d and initial model.
Fixed bug where the fill_value of spectra changed from nan to zero.
Refactor to allow fitting either fobs or f_syn.
Changed initial parameter range.
Using mean instead of median for wide band fits.
Fixed incompatibility with some versions of pyfits.