Fix a bug in using TLES env variable

[pnuu]

The behaviour of using TLES environment variable was changed in #129. This PR reverts the usage so that TLES='/path/to/tle_files/tle*txt" works again.

The question is, should the TLES=/path/to/tle_files" introduced in #129 work?

• Tests adjusted
• Tests passed
• Fully documented
• Passes flake8 pyorbital

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2c06303) 87.51% compared to head (97df4c1) 87.51%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #143   +/-   ##
=======================================
  Coverage   87.51%   87.51%           
=======================================
  Files          14       14           
  Lines        2210     2210           
=======================================
  Hits         1934     1934           
  Misses        276      276           

Flag	Coverage Δ
unittests	87.51% <100.00%> (ø)

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[coveralls]

coverage: 87.562%. remained the same
when pulling 97df4c1 on pnuu:bugfix-local-tles
into 2c06303 on pytroll:main.

[mraspaud]

LGTM! But I'd like this do be double-checked by @adybbroe

[mraspaud]

It's actually a feature, not a bug... but the new behaviour in 1.8 is actually problematic, so we are reverting this here.