iono_split_spectrum: hardwire dset while inverting for iono TS

[yunjunz]

Description of proposed changes

• iono_split_spectrum:

  • hardwire dataset name to unwrapPhase while inverting for iono TS
  • remove the template file input, so it can ignore the template inputs, such as the unwrapPhase_bridging if one turns on the PU error correction.

• defaults.smallbaselineApp.cfg: update correct_ionosphere reference, to be consistent with iono_split_spectrum.py help message.

Reminders

• Fix ionospheric corrections in MintPy #1189 (comment)
• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
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