`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

• [x] Pass Pre-commit check (green)
• [x] Pass Codacy code review (green)
• [ ] Pass Circle CI test (green)
• [x] Pass local test
• [x] Make sure that your code follows our style. Use the other functions/files as a basis.
• [x] If adding new functionality, add a detailed description to the documentation and/or an example.