astronomR 0.1.0

• Initial release of astronomR.
• Angular conversion functions: deg_to_dms(), dms_to_deg(), deg_to_hms(), hms_to_deg(), deg2rad(), rad2deg().
• Physical constants dataset constants_df and retrieval function constant_value().
• Gaia Archive query function get_gaia_data().
• Distance conversion functions km_to_Mpc() and Mpc_to_km().
• Early-universe thermal physics functions: photon_energy_density_fn_T(), photon_energy_density_fn_z(), photon_number_density_fn_T(), photon_number_density_fn_z(), Saha_Xe(), and soln_saha().