add option to save flat earth phase for interferogram and coherence generation

[IslamAlam]

Add the option in the GUI and GPT to save the flat earth phase for comparing it with other estimation techniques.

It has been tested on a Linux machine.

[lveci]

looks good. We're in a code freeze at the moment while we prepare the next release. I'll test and merge after snap 9 is out. thanks

[CLAassistant]

All committers have signed the CLA.