Enable expression of line spread function kernel in km/s

[chummels]

Spectrographs express their spectral resolution (i.e., the width of their line spread function) in velocity space. Currently, if one tries to use Trident to generate a Gaussian or Boxcar LSF, one has to specify the LSF in number of wavelength bins. A spectral resolution described in a fixed velocity will vary its spectral resolution in wavelength space across the spectrum, which requires changing the LSF kernel width over the width of the spectrum. But I believe the gaussian kernel convolution that we're using requires integral bin kernel widths, so we'll likely have to create different kernel widths for different parts of the spectrum and then combine them piecewise.