Make use of env variables free from satpy

[adybbroe]

This PR tries to clean up a bit concerning the use of environment variables and tle file handling. The old (mpop) legacy env variable PPP_CONFIG_DIR is removed. A new one is introduced instead: PYORBITAL_CONFIG_PATH. It is now also documented correctly and this also includes the use of the TLES environment which was not mentioned.

• [x] Closes #112 and #111
• [x] Tests added
• [x] Tests passed
• [x] Passes flake8 pyorbital
• [x] Fully documented

[codecov[bot]]

Codecov Report

Merging #113 (5a0ca49) into main (5590f65) will increase coverage by 0.60%. The diff coverage is 82.85%.

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
+ Coverage   87.04%   87.65%   +0.60%     
==========================================
  Files          13       15       +2     
  Lines        2054     2203     +149     
==========================================
+ Hits         1788     1931     +143     
- Misses        266      272       +6     

Flag	Coverage Δ
unittests	87.65% <82.85%> (+0.60%)	:arrow_up:

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

Impacted Files	Coverage Δ
pyorbital/check_platform.py	0.00% <0.00%> (ø)
pyorbital/logger.py	0.00% <0.00%> (ø)
pyorbital/tests/test_aiaa.py	100.00% <ø> (+4.76%)	:arrow_up:
pyorbital/tests/test_astronomy.py	100.00% <ø> (+13.33%)	:arrow_up:
pyorbital/tests/test_orbital.py	99.14% <ø> (+2.86%)	:arrow_up:
pyorbital/tlefile.py	93.12% <91.66%> (+1.78%)	:arrow_up:
pyorbital/tests/test_tlefile.py	99.75% <99.06%> (+1.74%)	:arrow_up:
pyorbital/tests/test_geoloc.py	100.00% <100.00%> (+5.66%)	:arrow_up:

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[djhoese]

FYI if you have other PRs you are planning you could drop Python 3.8 support and add 3.11 to the CI tests.