Background subtracted meta pixels in imaging demo

[hannahc243]

In the imaging demo, only the count rate fluxes (rate/kev/cm^2) are background subtracted and not the rate/keV. The remaining dictionary ``meta_pixels_bkg_subtracted" has the previous values for the rate/keV. This could lead to errors in calculating the background subtracted counts in an image.

  meta_pixels_bkg_subtracted = {
      **meta_pixels_sci,
      "abcd_rate_kev_cm": meta_pixels_sci["abcd_rate_kev_cm"] - meta_pixels_bkg["abcd_rate_kev_cm"],
      "abcd_rate_error_kev_cm": np.sqrt(
          meta_pixels_sci["abcd_rate_error_kev_cm"] ** 2 + meta_pixels_bkg["abcd_rate_error_kev_cm"] ** 2
      ),
  }