[srf_out] = SplineSrf(wls_in, srf_in, wls_out, [extend])
Convert the wavelength representation of a surface reflectance function.
Handling of out of range values:
extend == 0: Cubic spline, extends with zeros [default]
extend == 1: Cubic spline, extends with last value in that direction
extend == 2: Linear interpolation, linear extrapolation
srf_in may have multiple columns, in which case srf_out does as well.
wls_in and wls_out may be specified as a column vector of
wavelengths or as a [start delta num] description.