Adding observed_gsm property and hpix2sky, sky2hpix

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PR Summary

• Enhancement to BaseObserver
  A new observed_gsm (Global Sky Model) attribute is added to BaseObserver, which will offer a comprehensive, observed picture of the sky.

• Addition of coordinate helpers
  Two coordinate helper functions, hpix2sky and sky2hpix, are introduced to BaseObserver, facilitating the conversion between commonly used celestial coordinate systems.

• Improvements in base_observer.py

  • Import statements for newly added helper functions, hpix2sky, sky2hpix, and SkyCoord, are included.
  • The below-horizon mask is now generated using the query_disc function, improving the method used to define the visible and invisible parts of the sky.
  • Changes have been made to convert from Galactic coordinates to Equatorial and center on zenith, leading to more accurate celestial object positioning.
  • The observed GSM is generated by applying rotation and masking, contributing to more reliable sky data representation.
  • A new attribute for observed_gsm is introduced, offering direct access to this valuable data.

• Enhanced functionality in plot_utils.py
  A new function, show_plt, was included to call plt.show(). This extends matplotlib's capabilities in generating plots.