topsStack ionosphere network: select and pair the SLCs with different starting ranges

[yuankailiu]

Here are some proposed changes to the ionospheric phase estimation module for the topsStack processor under contrib/stack/topsStack

s1_select_ion.py: -sn option

• make -sn as floats to accept negative values for the southern hemisphere

• Add examples to --help message

stackSentinel.py: ion pairing between diff starting ranges

The changes are applied to the section: Select pairs of diff starting ranges to connect groups of different starting ranges to eventually form a fully connected ionosphere network.

• Fixed the indentation error on cnt += 1. It incremented even if we didn’t have pairs. This bug could cause the network to be disconnected. This was also brought up by @olliestephenson, and also in https://github.com/isce-framework/isce2/issues/499#issuecomment-1166623490).

• Review and re-write the pairing logic in this section to connect groups of SLCs with different starting ranges.

• Allow for pairing between diff starting range groups from both ends of the network. Such that, when pairing long S1A and S1B networks, beyond having a full rank matrix, we can have connections at both ends of the long S1A/B sequences. This was brought up in an offline discussion with @olliestephenson. The intuition/concern is that we could have some kind of divergence between the A and B ionosphere that we wouldn’t be able to notice without checking the consistency at both ends. But we are not entirely sure about this.