header check in case spectrum isn't found

get_low was throwing an error if no spectrum was found (because
then there was no associated header to access for information)
so now there is a check for that, and if there's no available header
then the slit_width is set to -9. (which should throw obvious enough
errors further downstream)
check that SpeX wavelength grids are actually at the same R

Because I need to do R-dependent smoothing, I've created a model grid
that is smoothed to the R-array of ONE SpeX spectrum, but if I'm going
to apply that to all the other BD spectra from SpeX I need to know that
the R vs. Lambda curves are the same for all the SpeX spectra.  So this
plots R vs. lambda for all 12 of my SpeX spectra.