update the IceSat-2 R package (OpenAltimetry --> EarthData)

[mlampros]

I'll add information in this issue as I update the IceSat-2 R package after the migration of the OpenAltimetry to EarthData

• available_nominal_orbits (Nominal mission orbits)
• available_RGTs (Reference Ground Tracks (RGTs))
• degrees_to_global_grid (Create a global grid based on degrees)
• getTracks (Get the ICESAT-2 Tracks)
• get_atlas_data ("atl03" is not yet functional but 'atl06', 'atl07', 'atl08', 'atl10', 'atl12' or 'atl13' work as expected )
• get_level3a_data(Get IceSat-2 ATLAS 'Level-3A' data for a time interval)
• latest_orbits (Extraction of the url from the Technical Specification Website)
• ne_10m_glaciated_areas (Natural Earth 10m Glaciated Areas (1:10 million scale))
• overall_mission_orbits (Overall Mission Orbits)
• revisit_time_RGTs (Revisit Time Reference Ground Tracks and Dates)
• RGT_cycle_14 Reference (Ground Tracks (RGTs) for IceSat-2 Cycle 14)
• select_aoi_global_grid (R6 Class to Select an Area of Interest (AOI) from a Global Grid)
• time_specific_orbits (Time Specific Orbits)
• verify_RGTs (Verification of the Reference Ground Tracks (RGTs))
• vsi_kml_from_zip (Utilizing Virtual File Systems (vsi) to extract the .kml from the .zip file)
• vsi_nominal_orbits_wkt (Utilizing Virtual File Systems (vsi) and Well Known Text (WKT) to access the 'nominal orbits')
• vsi_time_specific_orbits_wkt (Utilizing Virtual File Systems (vsi) and Well Known Text (WKT) to access the 'time specific orbits')

This issue is a continuation of #13