Remove the ppp_config_dir scene creation parameter for satpy>=0.26.0 compatibility

[mraspaud]

As of satpy 0.26.0, the scene creation doesn't accept the ppp_config_dir parameter anymore. This PR removes it, since the prefered usage is to path SATPY_CONFIG_DIR as an environment variable now.

• [x] Tests added
• [x] Tests passed
• [x] Passes flake8 trollflow2
• [x] Fully documented

[codecov[bot]]

Codecov Report

Merging #106 (54c3759) into master (0f87b0b) will decrease coverage by 0.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
- Coverage   93.58%   93.48%   -0.10%     
==========================================
  Files           8        9       +1     
  Lines        1668     1674       +6     
==========================================
+ Hits         1561     1565       +4     
- Misses        107      109       +2     

Flag	Coverage Δ
unittests	93.48% <100.00%> (-0.10%)	:arrow_down:

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Impacted Files	Coverage Δ
trollflow2/plugins/__init__.py	91.04% <ø> (ø)
trollflow2/tests/test_trollflow2.py	99.85% <100.00%> (ø)
trollflow2/__init__.py	66.66% <0.00%> (ø)

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[mraspaud]

duplicate of #97

[ghost]

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[djhoese]

FYI It is SATPY_CONFIG_PATH not DIR.