utils0: use more hardwired EARTH_RADIUS if needed

[yunjunz]

Description of proposed changes

• utils.utils0: use more hardwired EARTH_RADIUS if needed, to be more tolerant of potential missing metadata

• tests.objects.ionex.py: add test_plot_ionex() to plot an IONEX file easily in the command line, but did not call/test this function in the routine unit test.

• objects.sensor: add more Gaofen3 parameters

Reminders

• Fix metadata missing while subset using latlon on alosStack products #1256
• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.

[ditafaith]

Hello @yunjunz
I have run ISCE2 and MintPy in the same environment. Then ran prep_isce.py, but again failed to apply the subset function on MintPy. Your feedback would be very helpful.