[Discussion]: Store fitted model separately in WavelengthCalibration1D

[rosteen]

#162 implemented 1D wavelength calibration. In the initial implementation, fitting the input model to the wavelength/pixel pairs from the lines overwrites the input model with the fitted one, so accessing calobj.model will return an astropy model with the fitted parameters. This issue is simply to allow input on whether it would be desirable to conserve the initial model parameter guesses by storing the input and fitted models in separate class properties.