rads.xml:- Correct comment for drange_cg
- Add/update alt_gps orbits for j2, j3, 6a
- Update orbit info for ERS-1/2 and Envisat
- Fix add_offset for all SWOT ranges
- Set new ref_frame_offset* for SWOT (#195)
- Set proper limits for flag_alt_oper_mode (6a c2 tx pn)
radsxogen:- Correctly report maximum tie interval in xovers (#196)
- Fix handling of if/elseif/else in xml (#197)