gnss: rename JPL-SIDESHOW to SIDESHOW

[yunjunz]

Description of proposed changes

• utils.arg_utils.add_gnss_argument(): rename --gnss-source JPL-SIDESHOW to SIDESHOW for simplicity

• objects.gnss: use SIDESHOW instead of JPL-SIDESHOW or JPL_SIDESHOW for all variable names and values

• objects.gnss: add reference info in the code comments for GNSS_UNR/SIDESHOW, did not find citation/reference info for GNSS_ESESES, thus, did not add it yet.

• docs.references.md: update to reflect the current references noted in the smallbaselineApp.cfg file.

Unrelated changes:

• utils.network.simulate_coherence(): convert SNR from dB to 1 for thermal decorrelation

Reminders

• Fix rename --gnss-source from JPL-SIDESHOW to SIDESHOW #1228
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• Pass Codacy code review (green)
• Pass Circle CI test (green)
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