Sky spectrum needs proper units

[astronomyk]

The following code lines need fixing

If ATMO_BG_MAGNITUDE is given, the units of the EmissionCurve are ph/s/m2, and if ATMO_BG_MAGNITUDE is not given, the units are the original ones from the skycalc output, ph/s/um/m2/arcsec2. This means we get ridiculously high background fluxes.

Is it worth adding the test to the scale_spectrum_sb function?

   sky_mag = self.cmds["ATMO_BG_MAGNITUDE"]
            if sky_mag is not None and isinstance(sky_mag, (float, int)):
                lam, val = scale_spectrum_sb(lam=lam, spec=val,
                                             filter_name=self.cmds["INST_FILTER_TC"],
                                             mag_per_arcsec=sky_mag,
                                             pix_res=self.cmds.pix_res,
                                             return_ec=False)
            
            self.ec_atmo = sc.EmissionCurve(lam=lam, val=val,
                                            pix_res=self.cmds.pix_res,
                                            area=self.cmds.area,
                                            units="ph/(s m2)",
                                            airmass=self.cmds["ATMO_AIRMASS"])