add dem_error.py --dem-err-file and save_qgis.py --zero-first option

[yunjunz]

Description of proposed changes

• dem_erro.py: add --dem-err-file option to be able to specify an customized file name for the estimated DEM error. Fix adding a dem error file naming option: --outfile-demErr #971.

• save_qgis.py: add --zf / --zero-first option, same as tsview.py, to set displacement at 1st acquisition to zero. Fix support save_qgis.py --zero-first #749.

Reminders

• 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 adding new functionality, add a detailed description to the documentation and/or an example. [Skiped since these are minor modifications for not-so-commonly used options]