isce_utils: add crop_slc() and form_ifgram()

[yunjunz]

Description of proposed changes

• utils.isce_utils: add crop_slc() and form_ifgram() for more flexible interferogram manipulations.

• tsview: add exclude date in the saved text file header, to be more informative.

• circleci: use cimg/base docker image, to replace the ubuntu:bionic image

Reminders

• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• Pass local test
• 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.