Add tests and refactor trollstalker a bit

[mraspaud]

This PR adds a simple functionality test for trollstalker. At the same time the main functionality is moved out of the bin directory into the pytroll_collector package.

Another test is commented out for now, as this will be used in the next PR.

[codecov[bot]]

Codecov Report

Merging #141 (01ee58d) into main (5f154a1) will decrease coverage by 0.92%. The diff coverage is 73.68%.

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
- Coverage   91.64%   90.72%   -0.92%     
==========================================
  Files          27       29       +2     
  Lines        4115     4419     +304     
==========================================
+ Hits         3771     4009     +238     
- Misses        344      410      +66     

Flag	Coverage Δ
unittests	90.72% <73.68%> (-0.92%)	:arrow_down:

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Impacted Files	Coverage Δ
pytroll_collectors/trollstalker.py	70.80% <70.80%> (ø)
pytroll_collectors/tests/test_trollstalker.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[coveralls]

Pull Request Test Coverage Report for Build 5067010113

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 4816774831:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls