Support for Exo-REM model spectra and new features for FitModel

[tomasstolker]

• Support for Exo-REM model spectra with add_model, FitModel, and ReadModel.
• The (mean) spectral resolution is stored for each spectrum of an object.
• FitModel will use the spectral resolution of each spectrum to smooth the spectra with a Gaussian kernel before interpolating.
• Updated smooth_spectrum to smooth a spectrum for which the wavelength spacing has a uniform spectral resolution (fast) or constant step size (slow), which is also used by get_model.
• The absolute_to_apparent function to convert absolute to apparent magnitudes.
• The update_spectra function for updating the spectra of an object with a best-fit scaling and/or error inflation.
• Optional scaling parameter in FitModel that can be fitted for each spectrum.