Investigate ring elevations in Becker/Jarmak solar occs

[mace-space]

The rings:observed_ring_elevation values in the Becker/Jarmak solar occultation labels may contain errors that need to be investigated.

In the PDS4 Becker/Jarmak Occultations - OPUS database fields sheet, we noted that:

• Solar Ring Elevation = 90 – Observed North-Based Incidence, with a note "Would be – rings:observed_ring_elevation, but that should become negative after 2009 and doesn't, so there may be errors in the Becker/Jarmak labels for rings:observed_ring_elevation"
• Observer Ring Elevation = 90 + Observed North-Based Incidence, with a note "Would be – rings:observed_ring_elevation, but that should become negative after 2009 and doesn't, so there may be errors in the Becker/Jarmak labels for rings:observed_ring_elevation"

Currently the OPUS import code just uses the same method as Uranus Occs to avoid crashing:

# TODO: investigate the following. Would be -rings:observed_ring_elevation, but that should
    # become negative after 2009 and doesn't, so there may be errors in the Becker/Jarmak labels
    # for rings:observed_ring_elevation
    # def field_obs_ring_geometry_solar_ring_opening_angle1(self):
    #     return (90.0 - self.field_obs_ring_geometry_north_based_incidence1())
    # def field_obs_ring_geometry_observer_ring_opening_angle1(self):
    #     return (90.0 + self.field_obs_ring_geometry_north_based_incidence1())
    # This crashes so using same method as Uranus Occs for now (below).

    def field_obs_ring_geometry_solar_ring_opening_angle1(self):
        oa = self._index_col('rings:observed_ring_elevation')
        if oa is not None:
            oa = -oa
        return oa

    def field_obs_ring_geometry_solar_ring_opening_angle2(self):
        return self.field_obs_ring_geometry_solar_ring_opening_angle1()

    def field_obs_ring_geometry_observer_ring_opening_angle1(self):
        return self._index_col('rings:observed_ring_elevation')

    def field_obs_ring_geometry_observer_ring_opening_angle2(self):
        return self.field_obs_ring_geometry_observer_ring_opening_angle1()

Note: this likely affects the OPUS import, see SETI/rms-opus#1427.