support `save_qgis.py --zero-first`

[svenborgstrom]

Goodmorning, currently the "save_qgis.py" script doesn't support a flag like "--zero-first" (as in tsview.py) to set to "0" the first value of the resulting shape file. Is there a colleague/developer who can take care of this? Thanks, Sven Borgstrom.

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[yunjunz]

In a nutshell, it should work similarly to the tsview.py --zero-first option, by changing the reference date to the first date on the fly.

A more detailed description of the desired functionality is on the user forum: https://groups.google.com/g/mintpy/c/_BDhjO4-KNc.