Add SIGTERM handling to segment gatherer

[pnuu]

This PR adds SIGTERM handling to segment gatherer.

After receiving the signal, the gatherer will wait until all active collections have been finished and then exit the program.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 14 lines in your changes missing coverage. Please review.

Project coverage is 91.82%. Comparing base (80956c4) to head (3409378).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
pytroll_collectors/segments.py	35.71%	9 Missing ⚠️
pytroll_collectors/tests/test_segments.py	85.71%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
+ Coverage   91.23%   91.82%   +0.59%     
==========================================
  Files          32       28       -4     
  Lines        4417     3953     -464     
==========================================
- Hits         4030     3630     -400     
+ Misses        387      323      -64     

Flag	Coverage Δ
unittests	91.82% <71.42%> (+0.59%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[coveralls]

Pull Request Test Coverage Report for Build 11815651515

Details

• 35 of 49 (71.43%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+91.0%) to 91.035%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pytroll_collectors/tests/test_segments.py	30	35	85.71%
pytroll_collectors/segments.py	5	14	35.71%

Totals
Change from base Build 7932120988:	91.0%
Covered Lines:	4062
Relevant Lines:	4462

---

💛 - Coveralls

[mraspaud]

Looks good, just one styling comment.

[mraspaud]

How about we put this in the while statement also, and then the stop after the loop? (and probably use break after the keyboard interrupt).

[pnuu]

Adjusted in 3409378

[mraspaud]

LGTM!