updating documentation
interp_models returns array of -99s for unfound model; __call__ returns -inf in that case
added resolution arg; don't need unit change in interp check

Added an argument to give the data resolution if smoothing is needed.
In the interpolation check the data_wav units were being converted,
but that's unecessary because the units are converted in the previous
line.
added resolution arg; moved data wavelength unit conversion

Data wavelength array is converted to the model units in __init__
instead of at every interpolation