Add fallback when TLES environment variable not listing any files

[pnuu]

Fixes #74 with a fallback to downloading TLEs from internet.

• Closes ValueError in case of empty or missing TLE files #74
• Tests added
• Fully documented

[codecov]

Codecov Report

❌ Patch coverage is 97.36842% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.78%. Comparing base (0a7ede2) to head (c9e2909).
⚠️ Report is 66 commits behind head on main.

Files with missing lines	Patch %	Lines
pyorbital/tlefile.py	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
+ Coverage   89.58%   89.78%   +0.19%     
==========================================
  Files          17       17              
  Lines        2784     2809      +25     
==========================================
+ Hits         2494     2522      +28     
+ Misses        290      287       -3     

Flag	Coverage Δ
unittests	89.78% <97.36%> (+0.19%)	⬆️

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[djhoese]

I don't understand this change. What does this do? Reduce the number of lines by 1? Oh it adds a warning along with the debug log message?

[pnuu]

First the code needed to be refactored so that the "get TLEs from internet" case were a function. Then I added the warning when TLES env variable is given and didn't provide any files and the fallback of getting the data from internet is used.

[djhoese]

Thanks for getting this implemented so quickly.

My biggest worry with this, not fully understanding how users use this, is that someone (me) may want to turn this off, or may want to do the download manually, or may want to run this on a system that can't access the internet so this would fail anyway. Oh or they may be upset that a ton of files got downloaded when they were just testing things. Pyspectral does something similar within its configuration file.

[pnuu]

Thanks for getting this implemented so quickly.

Took only 4.5 years 😅

My biggest worry with this, not fully understanding how users use this, is that someone (me) may want to turn this off, or may want to do the download manually, or may want to run this on a system that can't access the internet so this would fail anyway. Oh or they may be upset that a ton of files got downloaded when they were just testing things. Pyspectral does something similar within its configuration file.

The other option would be to fail immediately if TLES didn't provide any data. I took the fallback route as the fallback method was in the next code branch.

There are only 9 text files that are downloaded. And as far as I can tell, the previously downloaded data are overwritten, so not much are accumulated.

[djhoese]

This looks good enough to me from the little experience I have with it. I assume if someone is customizing the "TLES" environment variable then they know that things might be downloaded. Should this be documented somewhere more public?

Oh one downside to this would be someone having a typo in their TLES glob pattern and then they get multiple copies but maybe that's OK.

[pnuu]

The idea of TLES env variable is that the user already has the TLE data locally and they want to use them. So maybe the fallback should be removed and a meaningful exception is raised? @gerritholl ?

[coveralls]

coverage: 89.742% (+0.2%) from 89.583%
when pulling c9e2909 on pnuu:fallback_empty_tles_env
into 67edf1c on pytroll:main.

[mraspaud]

Looks good, just one comment.

[mraspaud]

LGTM