Start deprecating automatic tle downloads

[mraspaud]

This PR starts deprecating automatic tle fetching from celestrak.

Automatic downloads are convenient when prototyping, however there are several reasons this should not be enabled by default, among which:

• Celestrak is throttled, meaning that a user can get banned (temporarily) after running automatic downloads for a while

• For operational environments, where this code can be run a lot, internet connection quality (plus the throttling above) can be a concern

• For consistency in automated tests, it is important to keep testing on the same data, not one automatically refreshed from the internet

• Downloading TLE from the internet takes a little time, which can be avoided by having a local TLE cache.

• Closes #xxxx

• Tests added

• Fully documented

[mraspaud]

pre-commit.ci autofix

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.46%. Comparing base (b10cfeb) to head (860e0aa).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
+ Coverage   90.43%   90.46%   +0.02%     
==========================================
  Files          18       19       +1     
  Lines        3011     3019       +8     
==========================================
+ Hits         2723     2731       +8     
  Misses        288      288              

Flag	Coverage Δ
unittests	90.46% <100.00%> (+0.02%)	⬆️

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[coveralls]

coverage: 90.421% (+0.02%) from 90.399%
when pulling 860e0aa on mraspaud:disable-tle-dl
into 10467c0 on pytroll:main.

[pnuu]

Couple of typos, otherwise LGTM